I am a high school history teacher. I think that what you saw with those children is something that is happening in all sorts of circumstances. Often when older people become parents, they see their children as special, more precious and more valuable than other peoples' children.
We have been seeing this in the schools for about 10 years. As these children are getting older and rising through the grades, the problems we see are getting worse.
I intend to retire before they hit high school age!
If I were in that situation, I would speak to the FA and then the most senior FA. After all, it's my flight also. If for some reason the most senior FA fails to act, I would demand that a report be made in writing to the airline with all relivant data including the seat numbers of the offending passengers.
Then I would speak to the parents. My line at this point almost always is "you are really going to enjoy these children when they are teenagers!"
At first they will think it is a compliment. One hopes that it will sink in.
If you don't complain to high heaven, this situation will only get worse. I promise!