I'm out of the kid show business now, but back in the day (last year) when I used to do them, I would actually open up with a very stunning (gimmicked) vanishing coin.
I then say to the kids that it looked like "real" magic, but we all know that it wasn't real, but we've all got really good imaginations, so for the next few minutes let's pretend that it IS real magic, and every time you see something that looks like real magic, clap your hands. Then during the act if they don't clap, I'll say something like, "that looked like magic to me..." Then they usually applaud.
Right after the opening coin trick, I ask them if they know a magic word. "On the count of three, yell out your favorite magic word." Of course someone always says "abracadabra," so I always pretend that I misheard them and that I thought someone said, "Have a banana." Then I do multiplying bananas.
At the end I make the bananas disappear, and I tell them, "on the count of three, say the magic word." Then I count to three, and I yell out "have a banana." Some of the kids will be smart enough to catch on and say "have a banana." Others will just say whatever. Anyway, for the rest of the show, that's the magic word I use. By the way, I got the magic word gag from a friend of mine ([edited] Roy) who probably got it from someone else.
Anyway, hope this helps.