Kirtles and dresses has the advantages of being fairly simple to make and covers a period of at least 600 years of basic children clothes . Embellishment, colors, fabric and to a certain degree length differentiates viking from late medieval.
I've been making kirtles for my extremely active and grubby little boy since he was 15 months. He is 4,5 years old now and has developed a marked taste of his own. He has some very strong opinions about things that itch,the color blue and what makes a viking. I'd strongly recommend consulting the child in question when buying fabric . A hated garment will be worn once or maybe twice.
Just a note of warning before you start ;
Despite what you tend to see at events and markets, there is little or no evidence of anybody but musicians and jesters wearing gores in a different color from the rest of the outfit. All extant scandinavian kirtles and garments are uni colored or mi-parti ; Skjoldehamn, Kragelund, Moselund , Bocksten , Söderköping , Ronbjerg St.Birgitta's cloak (thought to originally be an armless surcotte) and the Herjolfsness garments.
Herjolfsness no 44 kirtle pattern approx age 6 to possibly 8 (110 to 116 cm tall). Excell file with drawings to scale up. Based on an original pattern drawing from "Lille Margrete og andre børn i middelalderen". original by Malene Kort and Else Østergård.
Mi-parti is usually a kirtle divided down the middle of the front and back with each half a separate color. All gores would be the same color as the fabric next to it. Red gores for the red side, blue gores for the blue side.
My son's first medieval market the summer of 2002.
I think I spent a week handsewing this kirtle .It lasted one whole weekend before he outgrew it. It looks great on his teddy bear though. I learned a few lessons making that kirtle.
One of them was never make a round neck opening for a toddler. You get a nice wide neckline that nearly slips of the shoulder. Always make a slit down the front of the chest. Small children have bigger heads to body ratio than adults.
And the second one was that the recommended color choices should be anything blueberry compatible in addition to anything that blends with dirt, mud and food stains. You don't want to wash your wool that often and the child is thoroughly grubby within 20 minutes anyway. It's a long weekend / week(s) unless you take a certain pride in having the grubbiest of any species around. Mine is strangely water repellent as soon as the tent is up and he is properly dressed.
To make your basic kirtle or dress you need a few measurements. And you have to decide if you want the garment to last more than one weekend/week/season. "Yeah growing childe" will grow the second you have finished the garment. Toddlers faster than a 3-5 year old. .
There is no difference between girls and boys when it comes to growth bursts and basic clothes. Just the length of the garment. Children portrayed often wear the same clothes of the same length and it's usually something to grow in. Even the children clothes from Hjerolfsness are clothes to grow in judging by how loose fitting they are. One size, fits none.
If it's just a weekend outfit, you basically measure the child from shoulder to shoulder and add about 2,5 to 5 cm (1-2 inches) to that).
For a child done with growth burst every second week or so, it might be worth the gamble and make a kirtle that can last 2 seasons. At worst it's too big the first year and barely big enough the second year. You need to add a minimum of 5 cm (2 inches) to the shoulder measurement for comfortable width. Remember, children tend to get taller and thinner as they grow sometime after they turn 2-3 years old.
The arm opening needs to be about twice the actual size of the shoulder/arm circumference. Unless it's for a baby, I'd recommend a minimum opening from the shoulder seam to bottom of armhole opening of 15 to 25 cm / 6 - 10 inches. Giving you a circumference of 30 to 50cm / 12 - 20 inches. This gives you ample room when dressing and still makes it comfortable to wear. If you find it difficult to judge the necessary size of the arm opening, make the lower insertion point of the sleeve about halfway between the child's armpit and waist.
The kirtle or dress should reach somewhere between knee and ankle. Not too short and not long enough to step on. Midway between works fine for my more than active 4,5 year old.
With a bigger child, you can make the kirtle/dress long enough to almost reach the ankle. Belts are always an option.
Probably the only really easy bit. Do not cut until you have read about side gores and made a decision about them.
You need 2 rectangles ;

Width ; shoulder measurement + the extra cm / inches needed (with maybe 1cm extra for seams on each side).
Length ; estimated length after pondering advice above and about 1 cm of seam allowance top and bottom
If you are of the daring kind , you can make the front and back in one piece. Length will then be front and back length in one go + seam allowances.

Sew front and back pieces together and leave an opening of about 10cm in the middle.
And the easy part is now officially over. Grab child, measure circumference of head. Measure neck too if possible.

Cut a half circle at the top of the front piece 2 cm wider than child's neck. Now you need a long slit down the front. Make it about 10 cm /4 inches long.

Grab child, try to insert child through opening. If too tight on a small child, make longer slit. If too tight on a bigger child, look at the neck area. Possibly make longer slit too.

Remember ; you will loose about 1cm along the edge when you turn it back. And you'll need to make a slight curve at the back of the neck for most children.

Now might be a good time to line along the neck with some very soft linen or secondhand fur. Silk even. Do NOT use anything synthetic. It's chafes fast and children do tend to be very flammable. My son loves his wool kirtles and kaftan as long as he doesn't have any wool rubbing against his neck.
Either you are up for adventure or you aren't. There are 3 methods ;
Just insert a trapeze shaped sleeve straight onto the main body. Easy, simple and works.
Look at illustration. Insert trapeze shaped sleeve.
Moselund kirtle style armhole. You need to try the garment on the child. Decide if it's worth it. It also entails shaping the sleeves...See sleeves for more.
The gores are made up of 2 triangles partly or fully sewn together. To make these triangles, cut a square, draw a line with chalk from one of the corners across to the other (or be daring and do it all by unerring sense of where to cut), cut fabric. Sew the straight sides together (or not depending on choice below). Round of the bottom corners a little.
2 alternative methods of inserting side gores:
Make the side gores like the Moselund kirtle. All the way up to the underside of the armhole/opening. The gores shoud be about 5 cm/ 2 inches wide at the top and 20-25 cm wide at the bottom for the kirtle. 30-45 cm for the dress depending of tall the child is and if you use front and back gores.
Do not add extra width to front and back pieces if you choose this method.
Side gores like Kragelund/Bocksten and other kirtles with waist inserted gores. Insert gores between waist and hipbone. The gores shouldn't be too wide. Maybe 20-25 cm finished at the bottom for the kirtle. 30-45 cm for the dress depending on how tall the child is and if you use front and back gores.
The gores are made up of 2 triangles partly or fully sewn together. To make these triangles, cut a square, draw a line with chalk from one of the corners across to the other (or be daring and do it all by unerring sense of where to cut), cut fabric. Sew the straight sides together (or not depending on choice below). Round of the outside bottom corners a little.
Insert below the navel back and front.
Depending on how adventures you are, there are 4 choices for the tunic. 3 are historically correct. See bellow.
For the dress you might want to consider how historically correct you want to be. 2 extant dresses from the 13thC and some 13th and 14thC sleeveless surcottes do not have back and front gores (see Marc Carlson's site for more). You need to add extra width at the bottom of the side gores to compensate.
If you decide to go with front and back gores, follow instructions bellow and add about 5 cm to the bottom width.
Moselund ; You need at least 5-7 cm width at the top of each half gore to make the folds (depending on size of child) .
Sew them together about 5-10cm down the front. 10-20 cm at the back.
Kragelund ; open down the front, sewn up at the back.
Skjoldehamn, Bocksten , Söderköping , Ronbjerg, Herjolfsness...; 2 triangles sew together back and front
One piece gore. Method 4, isn't to be recommended. It makes the garment stiffer and often makes the kirtle look strange because it doesn't move well.
The joy of fitted sleeves....If you don't like fiddling, avoid fitted sleeves. Stay with the basic no nonsense trapeze shaped sleeve. It works every time.
Growing children and sleeves do not go together. You're basic choices are either saving enough fabric to extend the sleeves the second year ,and maybe if you are lucky use the kirtle for a third season even. Or you can make the sleeves about 5 cm/ 2 inches too long and still save fabric for next season. Just face it, sleeves are too long or too short no matter what. And if the sleeves fit, nothing else does.
This is where you can have fun. And this is an unisex option and optional.
The simplest kind of decoration is a chain stitched row of bright silk or a wool or cream linen thread around neck, wrists and bottom of the dress/ kirtle if you feel like it. Or if you need to cover up your machine stitches.
Strips of silk or fine wool fabric is another option. Along with ribbons or brocade bands of some sort hopefully not too historically incorrect .
You have either sold your child to the first willing stranger or you are ready to add to his or her wardrobe.