As we don't have kids I can't say for sure how we'd handle it. But Karma and I have given it a lot of thought and conversation. I like to be prepared and don't want to find ourselves 10 yrs down the road going uh...now what? My parent were very open in some ways and very hidden in others, which I think is part of why it took me so long to become comfortable with who I am sexualy.
We've decided that we'll answer questions based on their level of understanding. Given that they'll most likely grow up with a poly lifestyle, it'll probably be a bit easier for us.
Karma's brother is 19 and has aspergers. He seems to get things on a logical level only. So using him as a base, I'd offer to answer any questions and leave it at that.
My early childhood education experience leads me to follow the belief that as long as children are comfortable and not made to feel things are "wrong" they will adapt and not usaly have an issue. If they feel safe in their environment they'll feel safe in asking questions.
I'd say got with your intuition and knowledge of your own kids. Approach things as they seem ready. Encourage questions and make sure they know that all parties love eachother and no one is leaving anyone.