Family Friendly Onsen/Sento in Tokyo, Kyoto and Osaka?

I have a toddler and a young elementary school aged child, and I want to take them to do an onsen or sento, but they are not the same gender as I am.

I was wondering if anyone had any recommendations for where to go?

I heard that children under seven are allowed to accompany a parent of the opposite gender. Is that correct?

I was also wondering if people had recommendations for specific places to go. In Taiwan, they had family friendly onsen resorts with children's areas with things like small water slide areas.

Is there something like that in Japan?

If you have young children and take them to onsen/sento, what advice do you have? What has your experience in general been?

I speak of a bit of Japanese, if that matters.