If I know the person that I'm making a kilt for, I try to take into account body shape as well. When you're talking about a child, some kids have flat little buns and some of them have round cheeks. If their little buns stick out a bit, then I'd opt for a bigger pleats/apron differential to snug the kilt into the small of the back; if they're built kind of flat in the back, I'd put more into the pleats so that it snugs in less.

But, as Paul Henry says, even a half an inch difference in the splits amounts to only 1/4" on each side - not something to agonize over.