Another poly parent here (finally managed to name change). I've found that after 20 years, people tend to shut up with the accusations that my husband and I aren't committed to each other. I've had another boyfriend (for want of a better term) for about 18 years and he was the best man at our wedding (at which we obviously didn't vow to forsake all others; if people wonder why we got married at all, please see all the MN threads on why cohabiting isn't the same - an insurance policy in case of death, mainly).
Kids know we have various friends who have sleepovers regularly, and treat them all as avuncular/auntie figures (many of course are just friends, so there's no need to define the differences between friends and relationships). Boyfriend's kids are adults now and have come out as straight and monogamous, and sometimes babysit my kids like I did years ago.
I do keep my other relationships quiet locally because people I know have had complaints to social services, but a few mum friends know. As far as the school goes, if you're a committed parent who is present, stably housed, supportive etc, they really couldn't care less. The kid with a new 'uncle' or two daily is their concern.
My parents were worried years ago but have seen us being decent parents and have got used to it. And as someone upthread mentioned, we still only have 24 hours a day, work, kids, etc - it's not like anyone's getting that much sex... A much more common conversation is "you want to go see thatband/film/go camping? Why don't you take your other partner?" Much more like a large extended family or the village to raise a child theory, than orgies every weekend. Maybe when the kids grow up and move out...