Children and gender separated onsen

Hi, I’m looking for accommodation recs for Tokyo and Kyoto for my 10 year old son and I. He’s keen on capsule hotels and onsens, which aren’t family friendly, or friendly for a mother and son.

I’ve just found Dormy Inn which looks great. I haven’t had much luck with searching this sub (user error probably). Does anyone else have any recommendations of anything that could be enjoyable and involves sitting in water?

Thanks in advance

*edit – I understand no swimming in an onsen. I live in a country with a lot of water and just tend to call everything swimming. I’m just looking for activities that both my kid and myself will enjoy, I can’t do two weeks of just kid-orientated fun, but also, I have a really cool kid who wants to throw himself into Japanese culture.

by clittyglitter2000