I'm a midwife and some of things that get posted are just absolute ****e.
I don't go anywhere near those sorts of groups - I'm a member of 2 BSS groups that have very strict rules on only posting BSS ads and nothing else and just stay away from everything else.
OP - if you can get any identifying details on the child/parent I would report to either DOCS or the police, otherwise I'd just ignore it and let it go.
Some people just don't like being told that they're in the wrong when they think they're doing the right thing.
It might be easier to stay away from these pages to avoid this sort of moral conflict.