That's the nice thing I find about Yoshinkan: you can always count on the instructor to be hardcore!
We have seperate kids and adults classes, and we stress to them that their belt is not the same as the adults. The adults also occassionally help teach them with the sensei's, so the kids can see the difference. One time during ukemi lessons, the kids get too cocky, and my junior sensei chained about a dozen hiyaku ukemi (senshusei intensity), the entire class hushes up, and maybe one or two "whoa" around.
The adult class would sometimes demonstrate higher level stuff like jiyu-waza in front of them, and sensei even busted out a few aiki-wazas that he knew. I personally took a ushiro aiki-nage. That made the kids really get the impression of the difference in training quality, and subsequently they work much harder, aiming for entry into adult classes.
Gotta impress onto them that Budo is all about hardwork and determination man. If they think they can cheat their way in they will. Show them that rank means nothing in a battle, only true skills, and that can only be tempered through rigourous training.
True swords are forged from the hottest flames.