I replied in another post about kids... but here is some more...
I have a six year old that considers "mono" part of his family. He said he would be okay with him being part of his family as I have explained to him before that we can choose who we want in our family and that they can choose us.
My long time friend and ex-girlfriend is part of our family, she helped me through child birth and is a constant in our lives. I had chosen her to be so and she choose us. She had a girl friend for a good number of years that was also chosen by us, but when they recently broke up she choose to leave our family. I told him that it is her choice and that we will love her regardless of her choice.
I don't have older kids, but I go a lot by my gut feeling on things. I know that for me and my husband, "mono" is a perfect fit, and we have chosen him to be in our family for as long as he wants to be. We have chosen him for life and he us. My boy has accepted this and has welcomed him with open arms and all the love he can muster.
I am very proud that I have taught my child respect for individuality, different genders, races, sexual orientation and for himself. I did this by living it. It wasn't until I had a child I realized how important it is to do this if we are to raise good people to look after the planet and ourselves into the future.
Having said this I feel strongly that it is very important not to tell your kids about our poly relationships or struggles within it, unless some major hurdles have been crossed and there is a sense of comfort, continuation, respect, balance, and caring from all those adults involved. Kids don't need to know our "BS" at any age. It isn't their responsibility to take it on and deal with the stress of it, it's ours. cjj, I think you and your husband have tons of work to do before telling them anything!