We have 4 kids. 3 live at home. 18, 10 (almost) and 2 years. They all know we're poly. They all are ok with it. In fact the 10 year old thinks that if we weren't that would be weird.
It's all a matter of how you choose to act, and how you choose to teach them.
Obviously we weren't having sex in front of them when we were mono-and we aren't having sex in front of them now that we are poly.
We explained it as having more than one love. Pretty simple, pretty basic.
They've seen me kissing both Maca and GG. They know I go to bed in Maca's bed, and that I wake up in GG's (weird I know-but Maca has to go to work at 5:30 in the morning, so when he leaves I go to GG's bed cause I'm a TOTAL cuddle bug).
Never been an issue.
We have friends with kids-they keep it a secret, but they also don't live with their lovers either.
"Love As Thou Wilt"