Bring the birthday child up to help you with your next big trick! You explain you will turn him/her into a rabbit!
Show everyone your magic wand.
Place the wand in front of the spectator and let gravity unroll the wand revealing a digitally printed picture of a rabbit with a cut out of the face. The birthday child's face is seen through the hole.
Perfect comedy. Perfect audience participation. Perfect photo opportunity!
Packs small, plays big!