A lot of the postings here have dealt with the mechanics. Mixing things up each class is good: Some basics, grab releases, choosing teams for some competition (duck walk races, for example - sounds silly, but that was old school training for the hips, and the kids will love it - "quack quack".) bag work, 1 or 2 step sparring. Variety. Kids bore easily. Sometimes, teachers, too.
The other part of it is more important in the long run. Kids not only need discipline, they want it. They not only need to have expectations set, they want it. Then, when they meet realistic, but challenging, expectations, and are recognized for it, it gives them (this goes for adults, too) self-satisfaction and positive reinforcement. This motivates them for future accomplishments. And develops aa desire to please their authority figure - the instructor.
Everyone should have a little fun in class. But I believe the above paragraph is the key. I have seen all too often, kids with intermediate belts that can barely do a block without losing balance as their stance is so bad. Their front kicks, ax and side kicks all look the same. It is so dismaying to me - I can't even continue the list. It is due to the instructor simply not instructing, not setting expectations. If you don't expect much, you won't get much.
Set the bar higher, treat the kids right and they won't disappoint. And, you'll be teaching them more than kick-punch. That's what true martial arts is all about.