Kids are kids, until ages 13-14 they just want to have fun; after that you can get a little more technical.
However I think comments like "its more than most teenagers are up for" or "Gotta impress onto them that Budo is all about hardwork and determination man. If they think they can cheat their way in they will" are really unhelpful and in fact disrespectful to the children.
If they want to act like kids, they're welcome to stay in the kids' classes.
At our dojo, junior black belt is equivalent to adult 3rd kyu, and a junior black belt would need to transition to the adult classes to get further promotions. (That is, there is no junior nidan, sandan, etc.) The more senior kids are encouraged to train with the adults, but expected to handle themselves in a mature way if they do. Beginners who are 16 or older start on the adult track from the beginning.