Pecs, you are most certainly allowed to hold your opinion, and others are most certainly allowed to disagree with it.
I totally agree with what the original poster did, in fact given the circumstances I think she was extremely reserved and polite. When we are taking care of our grandchildren, we are in loco parentis, in other words we take the same care as a responsible parent would take of our grandchildren. A reasonable and responsible parent would certainly act in the way the original poster did, in other words they would seek to keep their child safe and free from harm, in as far as possible, a polite and reasonable way. I’m another ex teacher, and as such I very occasionally tell other people’s children not to behave as they are doing, if I feel the behaviour is harmful to another person or property. I would never stand by and watch my grandchild being hurt by another person, and I’m pleased to see most of the other grandparents on here feel likewise.
As for the very sad situation you describe pecs, I’m afraid that truly awful situations in life do not absolve us of responsible care for our children or grandchildren, thankfully the situation you describe is extremely rare, but other truly awful situations are not, people are coping the best way they can, and behaving responsibly despite these circumstances. This parent/career was behaving in a disinterested and irresponsible manner, she was responsible for the child and she needed to parent/care responsibility and not leave it to others.