No, here's what you say. If you mess up, just say "You know what kids, I just got real lazy and didn't want to use my real magic, therefore I tried it without magical powers, but this time, I'll show you with magic" It works 99.999999999% of the time!
That will NOT work. Eleven year olds are past the stage where they believe every little thing you say. They are beginning to lose their innocence. The best advice is what JGC2003 and KyletheGreat said. Read up on other threads in this forum and try and improvise in a situation. I can guarantee you that it'll end up being funnier and work better.