Besides just being a good example for your kids, there are some practical things that they can do with a calculator while shopping. I sorted them by age, but you may find some ideas work well for any age.
3 and under
1. Calculators make great play phones (I know they aren't learning about money this way, but sometimes surviving a shopping trip with a toddler is more important.)
2. Kids can learn numbers or just exploring what they can do on a calculator. Trust me, their teachers will thank you later.
4 and up
1. Teach them the symbols for adding or subtracting to help them do some simple math.
2. Have them figure out how many items are needed for your family.
3. Have them figure out which price is cheaper, and how much cheaper.
10 and up
1. figure out cost plus coupon
2. Price comparing on sale items or store brand
3. figuring cost per item in a package.
4. discovering if bulk packaging is cheaper
5. estimating total bill
Habits and Skills
If your kids learn these skills at a young age, and use them regularly, not only will they have a more discerning eye as they shop and grow, but they will also have a leg up on their peers who will struggle to learn how to do this from a book in school. Nothing compares to the real world application of math in the market place.
As your kids get older and more experienced, you will see that they depend on the calculator less and less, and that they start doing it faster in their head. If you want your kids to do it the hard way, feel free to have them bring paper and pencil, but I am giving ideas on quick ways to teach about money, not math processes.
If you have multiple children, they can compete in finding out who can solve the answer fastest, or who come s closest to the total bill at the end.
Shopping with kids can be fun and ALWAYS educational.
Do you have more ideas that you can do with a calculator in the store.