At this age, they probably won't be able to count out the money when you go shopping, but they will be able to do certain things. For starters, have them hand the money to the clerk when the total order has been rung up. While it might seem like a little thing to you, it will get them involved and will help reinforce the concept that you are paying for something (such as groceries). Also, if you typically pay for items with a debit card, begin using cash when they are watching to help them understand the concept.