My kids have always lived in a "poly family" though it was often not a sexual-poly dynamic.
My sister has lived with us most of their lives.
The last 7 years or so GG has lived with us.
We recently "came out" poly (September). All of the kids living with us are privy to this information. So is the rest of our extended family.
Honestly the kids reactions were... not. They didn't care one way or another who was sleeping with who. They already had all of us in their lives, already accept all of us as family (they've always called GG "uncle") and they already knew he was our youngest child's bio-father as he was the sperm donor that allowed her to exist!
We've found that life is ALWAYS easier for the kids with more of us here. They KNOW they can count on ANY of the 4 of us. So if mom or dad isn't available-it's ok, they have auntie or their uncle.
Ours are 18, 13, 9, 2 years and have lived in this type of environment their whole lives... though as I said not always were we open about who was sleeping where.
"Love As Thou Wilt"