I understand very well OP why this annoys you. Table manners was one of the social skills we taught our children, but a lot of today's parents see things differently.
I would advise you to say nothing at all about this, either to the children or their parents.
Children today are generally speaking allowed "babyish" behaviour for far longer than we were, or our children were. The thinking behind it seems to be, as far as I have been able to ascertain, that children learn the skills they need quickly if left until they or their peer group feels the skill in question is necessary.
Whether this works or not, I cannot say, but I can say that most children learn to use knife, fork and spoon correctly at some point.
Time to point out that there are better ways of doing things than eating everything with their fingers, if they are still doing it when they are around fifteen,
I can assure you that their parents will see any remarks you make right now about their children's table manners as interference.
On the other hand, if you feel you must do something about it, and your regularly have the children visiting on their own, make a game of it. "Today we will try to eat the way Grannie was taught when she was your ages. Sit up straight, elbows off the table and use your fork and knife."
If it is light-hearted enough and made good fun, who knows?It might work