After they've earned enough dollars, they can spend them at the Mommy Store every Saturday. Its fill with junk, I mean treasures from the dollar store and knick knacks from that I've collected. I've heard of some families putting activities to earn in their store. Activities like video games, family outings, TV privilidges.
I'll let you know how it works out.
You can make your own dollers, rupples, pesos, etcs at www.festisite.com/money