I wouldn't go into too much information. Maybe just relate that you and your partner care for each other very much and let the child ask any questions.
As for their father, you may have to watch out if he may try to use that to get the kids. From what I have seen, it was a bigger concern in the 90's and less so over time.
I am very out. (I even wrote an article for my local paper on polyamory.) I find that for the most part, people are more confused about it than judgmental. I have even had people get defensive over their choice to be monogamous even though I saw poly is not for everyone.