Oh my, I had seven children. Two boys, two girls, then three boys.
They went into the tub together in all sorts of combinations LOL. When they were all little, all four were in the tub for my convenience.
You have two excellent responses and they are right. When the children become aware of the gender issue, it is time to separate them.
The swim undies are great idea, especially if the children really enjoy their bathtime.
I can remember , at ten years of age, having my baby brother and sister in with me to bathe them for my mother. She was three and he was two.
When I grew a pubic hair or two and Polly noticed, she must have been closer to four by then, I asked my Mum if I could just lean over the tub and bathe them. Think she knew what was going on LOL
I think, for children two years apart, that seven years old for the daughter, should be the separation point as long as she is comfortable til then.
Best wishes and God bless
Grandmother Lowell
PS It is so refreshing to find a Mum who doesnt mind putting both children in the tub to bathe and play together...good for you.