With regard to children and young mothers, manners are out of fashion these days. I do not agree with letting children do as they like but I appear to be in a minority.
My husband is disabled due to a stroke. When we travel on the underground it is heartening that people are generally considerate and give up seats for him, and it's usually a young adult that's up first. However, I notice that young mothers expect their children to have a seat each and they never help.
He is very vulnerable and won't go into a supermarket on his own because of small children running about. Like the OP, I have been told, "They're only children" when I have suggested that perhaps they could mind they don't knock someone over.
Sadly, I go home with the thought "nasty little brats" in my mind.
I don't give up, though.