The most important thing for us is to not act as if we're doing anything "wrong". My quad had a long-term commitment in place before we told my oldest, who is 14. He had a little (negative) experience with polyamory through friends, but we had a LOT of long talks with him and hopefully he's feeling more secure and we tried to reassure him that this just means more love for him.
With the younger kids, we just try to reinforce that these people are in our lives for the long haul. None of them seem freaked out by the fact that we all sleep in the same bed when we're together. We discuss the possibility of moving onto the same property together, but we've passed on living in the same house for the time being. Every once in a while when one of the younger kids tries something sly like asking me (the pushover) for something that their parents have told them no about, I joke about being the junior wife and they seem to think that's funny. So far, so good.
I see it kind of like introducing step-parents into their lives--you want to make sure that everyone is committed to the happiness and health of the kids.