Lots of color. Kids love colorful, flashy things. Stay away from fire, you may have a little arsonist in the audience and it'll come back to you playing with fire. No cards, they won't understand them.
Well...I have to disagree with exodus here...
He is right about kids loving color and flashy things...but I find fire okay in situations where you will be on stage. Now if you are in someones home doing a birthday party it is best to stay away from fire...but on stage it is just fine. Go for it if you have it...flash paper, flash string, etc. They are great ways to produce items. And kids do like card tricks and do understand them...you just have to know what kinds of card tricks to do for them. They must be very visual card tricks which are easily understood. I do them all the time for kids...heck, even David Ginn does cards for kids!