When I'm making Kilts for Kids I try to make as large a hem as is possible. Usually 4". In a Wool Kilt this is usually plenty of height change room and the pressed in hem in Wool steams out pretty easily. Avoid fabrics like P/V as the pressed hem is usually permanent.

I then add a hidden pleat in both the outer and inner apron edges. At least one full Sett each.

Then if the child is at an age when growth in the waist is expected to be greater than 3" in the next two years I fasten the Kilt with Velcro. I use 3" strips to give about 4-5" of "give" in the waist. If the child slows growth the Velcro can be easily removed and straps & Buckles added.

Other than that I make a Kid Kilt exactly the same as I do a Kilt for an adult.