Okay, um... my ruling is the guns have to be kept very secret(so no blatantly walking around with them) and extremely well-locked-up. Toshiko probably puts a magic lock on stuff like that(she has her own dangerous items to lock up too) so the kids can't get to them.

As for why children are detectives, most likely because they're willing to help and Toshiko is generally okay with taking them on cases as long as there's not a corpse or something.