We stayed 1 night at Kusatsu Onsen hotel village. It's indeed a resort which offers many activities (table tennis, bowling, game arcade etc.) and a separate spa center with swimming pool and sauna. We didn't bother to step out the hotel until we had to leave Kusatsu.
The dinner and breakfast buffet were delicious too.
There is shampoo and bathing tools for babies in the ladies bath. I believe it's suitable for family with young children.