I want to make a "punching bag/kickboxing" cake and make the figurine move its leg up and down as if it was doing a leg kick. I'm wondering if anyone out there knows how to do this or what is needed to make the leg move. I've seen this done on Ace of Cakes but, I don't know how they make things light up and move on their cakes! Thoughts?
I would look at places that sell small robotics or something?
I would suggest making your figurine hinged at the hip with one leg (i.e. right leg) attached to a base. Under your base you're going to want a rotating circle (horizontal - like a wheel turned on its side; use a simple, quiet motor from a hobby shop - if you explain why you need the motor they'll probably be able to direct you, and you'll probably get a smile at the same time - anytime I go to the hobby shop or home depot for cake making supplies it always amuses the employees).
Attach a piece of clear fishing line to the top of your figurine and measure the distance from the hinge to the point where you attached the line. Put a small hole in your base with the distance between the hole and the attached foot being approximately the distance you just measured between the hip and the fishing line.
Attach the fishing line to the edge of your rotating circle. Remember that the diameter of your circle will determine how high your figurine kicks. Be sure that your rotating circle is completely to the one side of the hole you have the fishing line going through - if the circle is directly under the hole then the length of your fishing line won't change, and your figure won't move.
This was the simplest approach I could think of. The downsides are that you won't get a bend in the leg before the kick so it won't be terribly accurate, but it will get the point across just as well.
If you really want the leg to bend before the kick, I would play around with some very small magnets. Using the fact that magnets of the same pole repel, play around with putting one magnet in the foot and then finding the right position on the base and/or stationary leg to cause the foot to repel once it gets past a certain position.
For kicks and giggles you could make a little lightweight punching bag for the figure to hit each time it kicks.
I hope this is helpful - good luck!