For us, we are simply using teachable moments at this age. I don't recall the sks ever flat out asking. One of SS's friends had two moms and we were more worried about the way they wanted to be addressed (Mrs? Ms? First name?) than the fact that they were gay and raising a son. Once SS realized we didn't care, then it was a non-issue.
And it's hard when you're sitting in church and your denomination won't let a gay pastor serve but professes to be "open" to all people. You can come here...but only if you don't do x and y. Thing is though, the individuals in each congregation may or may not agree with the Bishop or General Convention rules so that's kind of the route I'd take. The church says x, but I believe y and this is why.
DD is learning there are all sorts of families. Just last night she said that she and SD have DH for the same daddy, but DH is SS's stepdaddy. I said, "Hon, your family is a venn diagram and one day you'll figure it out." And then we explained that DH was SS's daddy as well and that the man married to THEIR mommy (vs her mommy) was their stepfather...etc. I'm sure she'll have friends who are raised in various families and we'll just go from there if she asks. We've already talked about how people are different shades, but they're still all people.
But I think the "why I believe x" is important. It's accepted by many people, but not all. So why do YOU accept it, or not? Get personal.
I would also not assume she's never heard about it. If she watches TV or reads books or goes online or has friends - I bet there's a lot she has heard already on a number of topics. She's also 10, so if you haven't had puberty discussions with her, it's a good time to start. Many girls start their periods as young as 9 or 10. And that can be a segue into sexuality and any other questions she has.