Your best to P.M one of the mods if you don't get a response. Personally I never use anyone under eighteen, I was warned once about an changed the story. It was just a dumb mistake on my behalf, in the case of using cartoon characters, you'd have to make it very clear they were cartoon characters, but whether you'd still be able to post it I wouldn't know.
Technically a cartoon character isn't any less fictional than one of the characters in the regular stories which feature 'real people'.
there's a post made by Myriads (at the top of all the posts made by others) that explain the rules pertaining to the stories anyone posts. if that doesn't help clarify anything for you, then ask one of the moderators.
but from what i understand, no fiction whatsover posted should contain a minor. if it does and a moderator sees it, he/she will remove it.