Measuring a youngin... room to grow?

[The Wizard of BC]

At 12 years old and size 12 shoes this young man will probably grow to be 6 foot tall or more so a hem that can be let down later is a good idea. Avoid synthetic fabrics as it will be very obvious where the hem was when you let it down.

You can build in growth pleats which can be let out as his girth increases.

I would not go for velcro as you say that you want this to be his adult kilt eventually. I would use normal straps and buckles but make the kilt large to begin with. An apron which overlaps by two or three inches is not a problem in a young kid and is almost un-noticable. Especially if the kilt is pleated to the stripe. As he grows you can simply move the straps and buckles.