Art of Problem Solving is great - I think they take credit for the US Math olympiad team's performance. I use books from the same organization at elementary school level and find them very good and fun to teach. I would recommend going a level lower among the books published by them [1] to see if that is a good starting point for your child. Their website also has some placement tests etc. to evaluate the right level for a student.

[1]https://artofproblemsolving.com/