I don't have kids, but if I had a 5 year old, I'd sure get them some of these: https://en.wikipedia.org/wiki/Cuisenaire_rods

I've often thought about inventing math toys, e.g., group theory for toddler: S_3 = triangle toy, S_4 square toy, etc. I'm sure there are many advanced math topics that could be turned into toys. No symbols, no equations, just toys, but subtly you're getting kids familiar with important math structures.