My father & stepmother were in a very similar situation and it was always an open topic, never hidden or anything that had to be 'sat down' & explained. I can't remember exactly how they dealt with it, I was too young but I know it was something my sister knew from very early on.

My DH came along when my DD was 11 months old. We explained it to her as early as she was able to understand and it's never been an issue.

I think when they start asking questions about where babies come from is a great opportunity to explain (after whatever other explanation you choose to give ) that there's more than one definition of a father or mother.

For example, what we told DD was that some daddies are daddies because they help to make the babies, others are daddies who live with you and love you.