The Wizards approach is the fastest, I believe. Taking out the darts will make taper smaller. Then just wrap the kilt tightly around the boy and reattach the strap. Oh, changing the apron and under apron (and possibly the fringe) is needed to make a kilt for boy.

I guess you need to make it much shorter, too.

Redoing the pleats is difficult, almost easier to take the kilt apart and make it from the beginning.

Good luck and steady hand!