I have a six year old boy. He knows that we chose our family and they chose us. Starting with choosing each other to marry. We have blood family and chosen heart family.
My husband and I started our relationship in poly about 12 years ago and the woman I was with at the time we brought him into our relationship is still a big part of our lives. We are not sexual, but in every way we consider her family and love her dearly... he knows she is family also and sees Mono, my other primary, the same way.
There is no need for him to know that we are sexual. He doesn't know what that is yet and it will be discussed at his pace. I don't believe in telling kids stuff until it is something that they bring up, or it fits a conversation. For instance we recently went to the local Pride parade. We have been going for years and he has always been told that we go to celebrate having love in our hearts for all different types of people. This year we talked about it again before going. We told him that he can love whomever he chooses, just as we love Mono and our ex even thought they are not a part of our blood family.
He told us how much he loves them and that he loves us too and his friend at school... it was our opportunity to tell him the reason for the parade and also to fit a word in about the increased presence of Mono, my other primary, in his life and allow him to love him also in his own way.