I found this article this afternoon. Maybe it will give you some ideas.
It was on www.polyamoryonline.org
"Outing" Yourself As Poly to Your Kids
Contributed by Chias
The single most important thing to remember in this situation is this: If there's something to say, say it. We continuously repeat our own personal mantra over and over in our heads. You know the one I'm talking about. It goes something like communicate, communicate, communicate. When dealing with the adults in our extended family, we continuosly preach communication, but when it comes to our children we tend to be tight lipped and hope they won't notice. Big mistake. Children are growing increasingly intelligent as the generations progress, and now more than ever we need to be leading by example. If we want our children to be open and honest with us, we had better make sure that we return the favor. If you've invited one or more of your partners to join your family, and they're living under the same roof as your children then chances are that the kids have already begun to wonder. Someone once told me that the truth is usually much less evil than anything we can come up with in our head. This can hold true for children as well.
Don't make the mistake that many poly family's make when dealing with their children. Don't wait too long to tell them the truth. How long is too long? If you've got your partners living under the same roof, and you haven't been completely straight forward with your kids, you've waited too long. As I've stated, children are growing increasingly more intuitive as the generations pass and we should never underestimate their intelligence, or their imagination level.
One member of our forums related a story to us regarding coming out to his kids. In reflecting back, he and his family realized they had waited too long. When they finally did sit their family down and explain it to them, their daughter already had seen things that had set her mind in motion. Apparently, she had witnessed a kiss between her mother and the other male living in the house. This type of thing is bound to make any child start to imagine the worst. Is there a separation or divorce coming? Don't my parents love each other anymore? They eventually explained the entire situation to all of their kids, and everything worked out for the best. This might not always be the case.
Despite the steps you take to be discreet in your relationships, at some point in time one of the kids is bound to hear or see something that reveals the truth. Take the time to sit them down and explain the situation to them before this happens. No matter their reaction, you'll at least know that you were honest and open with them.