they want to eat up front and how many they want to save. Then you could give them some interest rate, compounded every 10 or 15 seconds (so they see the effect of long term savings over a fairly short amount of real time).

They would see how after a while they were getting bigger and bigger interest payments, and their pile of m&m's would grow quickly. Seeing it happen in front of them would probably be more meaningful than seeing the output of a calculator.

And if you want to really make it realistic, your wife could play the role of the IRS and take a hefty percentage of the m&m's they end up with as long term capital gains tax (taking more from those who saved more at the beginning). Wait...that might not teach them what you are aiming for.