Every street I've ever seen like that has been an obvious "shortcut" street that they'd like to do a little traffic calming on during rush hour. For all pedestrians, residents of the street, etc. The kids are mostly at school well before 9am.

The only traffic calming I've ever seen around schools are flashing reduced speed limit signs, anyway.

I'm sure your ticket wasn't valid if it says 9:10 and the regulation was in effect from 7-9am, but I doubt children were the reason the sign was put in place, in any case.