There's no point in attempting to mold your child into a little Bill Gates, or in running your family like an accounting firm. As in everything you discuss with your kids, your role is to satisfy their curiosity in an honest and age-appropriate way. Dealing with money is such a natural part of life that you don't have to set aside extra time to talk about it. Instead, you can take advantage of situations that crop up every day. Your ultimate aim is to turn out independent adults who know how to manage money and have a healthy regard for what it can and cannot buy.
Abstract concepts are beyond most preschoolers.
They focus on the concrete, and so should you, showing them what coins look like and how they can be exchanged for other things. Most children get their first look at the world from the seat of a shopping cart. So learn to make the most of the opportunity, and steer clear of some nasty scenes as well. At the checkout counter, each kid gets to keep one thing from the basket. They won't bother if they know the rules, and know you'll stick to them.