My husband (Maca) , boyfriend (GG) and I live together.
I have a daughter who is now 20. She's known GG since she was a year old and Maca since she was 6.
Maca has a son who is 16, he's known GG and I since he was 2.
Maca and I have a son who is 12 and GG was the sperm donor for our 5 year old daughter.
We've been officially poly for nearly 3 years and been out the whole time with kids, extended family and close friends. The guys both work and have been out at work for the last 2 years.
Honestly, I can't imagine lying about something so significant to the kids.
Their lives have been significantly involved with GG and knowing that he's always going to be an integral part of our lives and family has been a good thing.
Additionally, after telling everyone we were poly and explaining our dynamic, my sister, stepbrother and several friends have "come out" about their own polydynamics and my fil came out as a swinger. So, the concept isn't foreign to the kids or the extended family.
As far as new lovers, we do have a rule that until a relationship is established as being long-term commitment-they aren't brought around the kids. This was agreed to after several potentials met the kids and the kids couldn't understand why they subsequently left their lives shortly there after.
There are by the way other threads regarding this topic if you do a search on kids/poly or children/poly.