I bribed them; told them they would get chocolate covered goji berries if they came up with a better project than just turning on a light or turning the motor. So they did. Here is what they built:
4 year old: The snap circuits pro is easy to make for you and you can make what ever you want BUT you have to make a circuit.
6 year old: Circuit means it has to circulate. That means go in a circle. The circuit has to be complete. It will turn out the way you want, just look for the numbers and letters and follow the instructions.