I had a similar situation a few years back on a flight from LAX.

The crew didn't do anything following my comments, seemingly because the parents were asleep. I went back into the cabin and woke up the child's mother myself and asked her to take control of her child as he was disturbing the rest of the cabin. She looked pretty sheepish to be fair.

If an adult were causing disturbance to other pax the crew would speak to the individual themselves. Clearly the crew aren't there to discipline pax children but they should be given clear instructions that they are to wake up parents if their children are causing a disturbance to others.