Do you have an inexpensive calculator that rarely gets used? If you have little ones, you can put that calculator to use. Give it to your littles and let them have free reign in playing with it.
Children love to punch in the numbers and see what comes out. You can even keep it with their toys. Don't let them have a clue that it's educational.
By the time they get to school age, they will not only be comfortable with using the calculator, but will have figured out how to use it - all on their own and in a natural way.
The beauty on this is that it is an inexpensive trick that requires zero time from you as teacher.