OP here, thank you for all the great ideas!
We are five days into our Fiscal Fast (only buying needs, not wants) and there was some whining at Target last night (they wanted to get a little something while we were shopping for groceries), but for the most part they are cooperating well.
I told them that at the end of the month - if our Fast is successful - they would each receive $20 to spend or save, and that incentive is helping. I will talk to them about Spend/Save/Donate before that happens.