Personally, I think white uniforms are impractical on kids, and medium or lightweight ones give the observer a far better view of the wearer's underwear than I like to have.
White shows dirt too well. I do think that everyone under black belt should have to wear the same uniform (hence the term), but I would go with black--it looks cleaner and thus wears longer, so it's cheaper in the long run.
I chose black for ours too for the same reasons, young and teenage girls as well as older women much, much prefer black Gi bottoms if possible. Black looks every bit as smart as white if not more so. Would recommend it if you can have them.