Ask HN: Where to find good problem sets for elementary mathematics?

I’m coaching elementary math / CS to a few kids of age 9-12 from my extended family and I often struggle to find or invent relevant problems for them to train on. Do you have pointers to problem sets that I could use for this purpose?

Concrete examples of topics for which I could use problem sets: computing angles in elementary geometry fractions solving linear equations combinatorics unit conversion