My fiance has three kids and I met them all a month after we started dating. At the time they were 2, 3 and 5. Sounds like your exs main concern is her not recognising him as her dad - maybe he needs a bit of reassurance from you that this will not be the case and also a plan that you will clarify with the child who is who.

I know that it seems common sense to us but I was surprised the other day when my stepson (5) starting telling me something about his "other mum". I had to explain to him that he only had one mum and whilst I love him dearly I am not his mum but his stepmum. Sometimes kids dont get things but it is not apparent straight away.

On a side note it is not actually any of your exs business who the child has contact with whilst in your care and vice versa unless there are abuse or endangerment issues or there are court orders to the contary.

Hope that helps