Teaching kids is completely different from teaching adults. I've been avoiding it but Uncle is really good at it but it took him 5 years to master. There are certainly tricks to the trade. It helps to be calm and decide you will never raise your voice! Realise you can't keep them all on track 100% of the time and struggling with one kid might mean losing the rest!
Uncle taught me that kids are used to doing nothing for short periods so it sometimes works to sit the class down & have two up at a time while the others watch & wait their turn (vs. the entire class errupting into chaos while you try to concentrate on two kids for 30 seconds). I've been watching DVDs by Melodie Shuman
Shuman Concepts, Inc. She developed the Little Ninja program and has lots of tricks with kids. It's really worth getting your hands on her stuff but meanwhile a couple I liked were: getting the kids to demonstrate a "1" kick on a scale of 1 to 10 (where 1 is a "limp noodle kick" and work them up to a "10". She also gets them to repeat key words after her a lot in a big loud voice to keep them engaged. And she uses subtle competition eg. "who thinks they can be first to get into a perfect zenkutsu dachi?" & let them raise their hands until they're all engaged & then start a bit of a competition. Also, you need to learn their names!!! All of them. It's a powerful thing to have your name used in front of the class when you're small. Finally, heaps of confidence (even when you don't feel it) and positive affirmation. Best of luck!