Ask HN: Challenging math software for 8yo kid?

She greatly enjoys mental arithmetic and I'd like to find some software that challenges her. There's loads of stuff out there but I don't know whether any of it is good. Ideally looking for personal recommendations.

For context, the mental arithmetic she does is stuff like adding 3 digit numbers, multiplying 2 digit numbers and some basic algebra. She continually asks me for math problems but I'm not sure where to go next.

Thanks in advance.