Use a book appropriate to their age.
For children aged 5 - 11 (primary) I would suggest the series of Books Beta Mathematics 1 to Beta Mathematics 6. The books I have are published in 1980.
The mathematics in these books is harder than that currently taught. But, I put that down to current teaching methods and the baby stuff that is taught.
Certainly it is very difficult finding a suitable book to use for ages 5 to 11.