I don't think any man should be in a bath with a 3 yr old child, especially when the child isn't his. There's absolutely no need to get into the bath to wash the child. I have 1 daughter myself who's nearly 6 and have bathed her many many times but never felt the need to get in with her and can only imagine it making it harder to bath the child!! If I even thought there was another man ''helping'' to bath my child I'd be concerned, you just cant be too careful these days.
This is just my opinion but it seems to be the general consensus, I know this is an awful position for you but I really hope it was innocent for the childs sake. At worst you will lose this ''friendship'' you have but maybe the lady involved will appreciate your concern. Is there any way you could ask about bath rituals for the kids etc without revealing too much??? If it was me I would be asking questions and probably telling what I'd seen, maybe try telling it offhandish or something to judge a reaction, maybe tell the story about another friend to judge her reaction???
I happily had baths with my daughter (and son!) until she/he reached 3 or 4 years of age. It's a wonderful way to bond with a child. However, I am talking about my own children.
In your situation, I would be most uncomfortable. Nevertheless, I think this needs to be approached with tact. Is the new b/f expected to get the children undressed for bed? If yes, then he is already being given the green light for the level of intimate contact which is associated with such action.
There are clearly only two possible explanations:
1. That the new b/f is acting in sexually-innocent manner, but trying to bond with the children, in which case any external investigation may well end up in legal action, ruining both his life and your friendship with the mum.
2. He is acting in a sexually predatory manner, in which case he should be barred from
any contact with the children.
In other words, he's either a pervert or he is not.
No one on here can possibly answer that, given the available facts. My own view is that it is incumbent on you to establish, with some urgency, the facts of the relationship between the b/f and the children. Jumping to conclusions invariably makes matters worse.