we took hiro to his first onsen when he was between eight and nine weeks old. we went to a family bath, a private bath which usually doesn't have a rotenburo, but it's really nice to have a helping hand (my husband) when washing a slick wriggly baby. once he could sit on his own, i could take him with me to the women's bath (or yoshi could take him to the men's bath, though he didn't) and wash him up. the one thing you have to remember (in my opinion) is to rinse off your child with 'fresh' water after getting out of the onsen for the last time. you should rinse yourself, too, i once got a rash from an onsen (eeewwww, gross) and a friend told me she always washed off in the shower before leaving the bathing area. good advice!

oh! i feel like i should add something. one should probably only take their very young baby to an onsen if their baby likes to take a bath! many babies love to bathe, but some do not. we were lucky in that hiro loves to take a bath.