[Shacklz]

I'm definitely by no means capable of giving proper advise here as it's been a long time since I've last interacted with kids of that age. But one anecdote:

When I was in the early teens (so quite a bit older than the kid in your case), I got my hands on a book about set theory, and it absolutely blew my mind. The concept of countability, axiomatic definition of functions and so on really gave me a completely different perception of math. Up until then, math seemed to be something that follows nature, so to speak, three plus two makes five because if I have three eggs and two eggs its five eggs or whatever primary school taught me. I remember back then that I'd wish that some teacher would have made me aware earlier of a more formal, axiomatic approach to math and all that, that there is a more fundamental basis to it than "that's just what it is". It really furthered my interest in math, and while I ultimately eventually moved over to CS, it definitely had quite a fundamental impact on me back then.

The particular book I've read was in German (I still have it) and unlikely to be ideal for a 5 year old; just wanted to give this little personal anecdote somewhat related to your question.

[Beijinger]

The kid does not speak German. But I do. What was the book?

[Shacklz]

Oliver Deiser, Einführung in die Mengenlehre