I am often surprised that people do not voice their concerns directly to offenders (parents in this case). One kick or scream is OK, but if it does not stop, get the parent and ask to take care of their offspring. And, of course, this was an UM situation and should have been handled as such, especially if the children were indeed disturbing other passengers. Surprised that FAs did not address it with the parents. What's the point of complaining to other paxs?
P.S. Have two kids, would never leave them alone, neither in Y nor J/F, even though I am quite certain that they would behave well anywhere.