It is not OK to tell the kids you disagree with the parents about parenting or about other serious life decisions. It is just as bad to be critical of the parenting of others to their kids or to your own kids, for that matter.
Other parents do not owe you an explanation for their actions and decisions, and almost certainly you do not have all of the facts. You have no business even evaluating their decisions unless you are exceptionally close, and you have no business ever informing the child that the parents are wrong. Regarding the situations you describe, about the most you can do is talk to the parent, but that is unlikely to have an impact.
In the vast majority of the cases, "Mind Your Own Business" is the proper starting point. We are becoming a busy-body culture, worrying more about others and not enough about our own affairs. The chances are pretty good that you have some parenting issues yourself that you are blind to, or you have some strategies that others might think are not very good. I remember once being berated by a mom for giving my child sweets. This mom's child was the schoolyard bully.
This does not mean that you can't be there for the kids, and love them, and support them. I realize that doing so without being critical can be difficult. And you can almost certainly model good parenting, with your kids and with them. But keep your mouth shut about the parenting.
My wife grew up in a troubled household, with alcoholism and affairs and lots of conflict. She spent a lot of time with her god parents, who were close to the family. She learned from the godparents how good parents should behave. The god parents were never openly critical of the parents to my wife until many years later. But their home was always open to her, and they supported her during the rough times. Had the god parents been critical, that criticism would have come out in an argument, and my wife would have been forbidden from seeing the god parents.
Think of it this way. Bad mouthing the parents and offering unsolicited advice and opinions are not going to change their behavior, and are likely to get you removed from the lives of these children. You can do more good being a silent, positive, supporting presence than being an absent critic.
DISCLAIMER: If there is an issue severe enough (abuse or serious neglect) that the welfare of the child is at stake, then talking to the parents is unlikely to be productive, unless you have a really solid relationship. More likely, a call to the local child protection agency is in order. But none of your examples sound remotely like that.