Yes you can take your kids with you to the hot tubs but if you are talking about the massage, I don't think that's advisable.
There are age limits for hot tubs
They don't have a "day care", so if you take them, they have to stay with you.
I believe there are rules set by the hotel, but then also use your own judgement. How old are your kids? Would you take them to a Japanese onsen? If so I think you should be fine. Note that the hourly hot tubs are still private.
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