No-one should put pictures of other people's children on the internet without first asking permission from the child's parents.
You don't know the circumstances of every child in the group. As Blazey said, some people do not want a violent ex partner to trace them and pictures on the internet may lead to them being traced......possibly with tragic consequences.
I would have no problem with one or two photos of my children being posted but I would not put group pictures on which showed other children without their parent's consent.
__________________
Old aunts used to come up to me at weddings, poking me in the ribs, cackling and telling me
'You'll be next.' They stopped when I started doing the same to them at funerals.