I didn't, but I could have, made the drill cards out of triangular pieces. A drill card would contain 2 of the 3 related numbers on one side, with the third as a blank or question mark. On the other side the full three were given so children could be challenged by others to come up with the third number in the set, or could work on their own. I know now that I should have made similar cards, but with all three numbers on one side. Paying attention to this distinction between numbers early on can be very useful later.
Children can learn to make an easy check of answers to simple or complex questions.
Children can soon work out the statements about even and odd numbers that always will be true for the whole numbers they work with. I believe it is good for children to know that math has such simple ideas that always are true. It can help to think of odd numbers as being an even number, plus one more.