I introduced L and E (3 years old) to money recently. They already knew the concept of money (taking it to the bank, buying stuff, etc..), but were not familiar with the coins and their values. I promised them that they could use their plastic play money to play 'Grocery Store', so that is the first thing they asked for the next morning, even before breakfast. It was really simple to set up, and they've been requesting the 'Grocery Store Game' all week...