Ask HN: Fun math resources for young kids

Our four year old has just become super-interested in numbers. All of a sudden they want to do lots of counting together, and are asking questions like: "why does 12 have a one and a 2?"

I told them about prime numbers earlier this week, and now I'm fielding questions about prime numbers that have pretty much exhausted my knowledge.

For obvious reasons, I'd love to encourage this interest in numbers, if it stays fun. I could really use some pointers to activities, books, etc about counting and math -- ideally, targeted for preschool age -- but stuff for later would be great, too!