Set up a grocery store using the groceries you have in the house.
Instead of recycling boxes of cereal and other items use them in the grocery store. If you are using canned goods and want them empty open them from the opposite end and put tape around if there are sharp edges.
Make everything $1 to keep it simple, especially for younger preschoolers. If they have 5 items, they can count out the items and pay with 5 $1 bills. This will promote counting and simple math.
For the older kids add the bigger bills and let them count out the correct amount. If they are the cashier teach them how to give back the correct change. By keeping it simple they will be learning about money and counting all while having fun!