For this, you'll need to ensure the children have rulers that are split into cm and mm (Even inches if you want to make it a bit more complex).

1. Give the children a starting number (e.g 13cm) Tell them to put their finger on it.

2. Tell them to add or subtract numbers - start slowly at first, but get faster...

To make it more complex for older children, you could use the mm as decimals, or even as fractions (for example, add 1 and 1/2 - 1cm, 5mm.)