I suggest instead of buying an "official kid's kilt belt" (that I think you will find likely to be of insufficent quality), that you take a look at USA Kilt's selection of regular belts (which should be proportionately the right size for a thick "kilt belt" on a child) with a sturdy celtic buckle. You will need to shorten the belt itself but you will have a buckle that he can use as he gets bigger.

Just an idea

Cheers

Jamie