Before they can do this, they should be secure doubling up to 10. (10+10)
From this, they should be able to see that halving is the opposite, and so should also know halves of numbers up to 20.
Here are some good games to reinforce these basics:
The children enjoyed using this machine to test themselves:
Doubling and halving machine
When teaching the children to double and half two digit numbers. We encourage them to split the number into tens and units. Double or half the tens, then do the same with the units. We call this a doubling or halving diamond.
When you try halving, use only even numbers to start with.
Here is a good game to practise both. You can change the level of difficulty.