Try doing a search in the tags for "parenting" "kids" etc. There have been discussions before about your topic.
Personally, having been poly IRL for 16 years (ish?), and having a 9 year old, I tell him I love his dad and I love my live in boyfriend. He recently found out that my gf is also a love of mine and more than a friend. I tell him that my love has no limit and I chose to express that. I think he thinks all my friends are partners, lol.
Now that he is older I tell him that not all people agree with me, but I am not worried because I don't see how I live as a negative in his life or anyone else's in our family. So they just don't know about it. He's a smart kid. He has found his own way with that info. He calls my bf his "best buddy." A term he came up with. He explains to everyone that he is lucky because his best buddy is also his roommate. Also came up with that.
Kids don't know details or care to know. To them its all normal because its a clean slate for them. Its only when adults make it a big deal or treat poly as abnormal (or anything for that matter) that it becomes so. So I don't apologise for who I am and don't make his life seem abnormal by acting as if it is.