- Every child orders anything desired.
- We note who spent the most and how much that is.
- Every child who spent less gets cash equal to the difference between their own order and the big spender's order.
- Alice spends $6 on ice cream.
- Bob spends $5.
- Charlie spends $4.
This works if the children really do compete to spend the least. Depending on the kids, they may get confused and ignore the game. They may also decide they care about ice cream more than cash (and ignore the game).
The worst outcome (for me, the ice cream treater) is if the one of the kids really understands the game and clues in the others. They make out like bandits if they cooperate like so: one kid orders for everyone and runs up a huge bill. The others spend zero. Then the one buyer trades ice cream for cash. In the above example, Alice spends $15 for everyone's choices, Bob and Charlie get $15 each, and the three kids then divvy up the money evenly. Each gets $10 plus ice cream.