Linen tunics / dresses has the advantage of being fairly simple to make and covers a period of at least 600 years of basic children clothes . The basic shape is like a T-shirt with or without side gores.
To make your basic linen tunic 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.
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.
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 tunic should reach somewhere between mid-thigh and knee . Or if you want to take the chance of getting 2 seasons out of it, shoulder-knee is a good length.
For a linen dress, make it long enough to reach between the knees and the ankles. For bigger children, almost ankle length works.  
Probably the only really easy bit.
Tunic : before you cut, decide if you want side gores and what kind of gores .
Dress: decide what kind of side gores you want.
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
If you are of the daring kind , you can make the front and back in one piece. Length will then be front and back lenght in one go.

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 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.
No side gores is a perfectly respectable solution for the tunic. Make slits up to the top of the hipbones instead.
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.
2 alternative methods of inserting side gores:
Make the side gores like the Moselundkirtle. All the way up to the underside of the armhole/opening. The gores should 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.
Just insert a trapez shaped sleeve straight unto the main body. Easy, simple and it works.
A simple row of chain stitches with white or cream linen thread around the neck and the wrist usually looks good. And if you have machine sewn the rest, now's the time to cover up