I'm making adjustments to the nickel game as I've been going along. Just in case you are tempted to institute something like this at home, you might benefit from some of these tips and changes I've made in the last couple of days.
When dealing in the 5 and under crowd, tangible is best. I now own a prize box, that is full of goodies the kiddos are gawking over. They each have their eye on something specific that they are working toward. Though the things in the box are of various values, they can pick a prize when they get to 20 nickels. It keeps it super simple so they always know they're trying to get 20, then the get to pick a prize. It also allows us to continuously be counting to 20... in a way that they don't even know is sneaky learning!
Oh, and the other rule is that you can never lose more than 3 nickels for the same offense. Remember, the goal is that they earn them, not lose them (unless it's necessary!). So a minor offense is losing 1 nickel, a moderate offense means you lose 2, and severe offense warrants losing 3, but never more.