Yeah, a lot of shows are like this. It might have something to do with child labor laws, but I doubt it because children do show up in TV shows. My other guess is that adults, especially those who are professional voice actors, will likely be able to do more with their voices. And, if you think about that, that's probably why many male characters that are young are voiced by women. The pitch of an adult man's voice wouldn't fit very well with a young male character. The only two examples if this I can find are Timmy Turner and the godly Jimmy Neutron, but I'm positive you'll find more if you keep digging.