me and my dad teach some little kids after school.. and they can easily get bored. basically, you have to be very creative. make up games and activities that teach karate skills, like 'sensei says'. make karate "fun" with lots of moving and yelling.
The more fun they have, and the less they get bored, theyll come back for more.
Watch some tapes on teaching kids, and ask them what they would like to do. You'll think of a fun game that will be special in your dojo only, and they'll be learning at the same time. good luck!!