The current method is, I believe called 'controlled crying' It seems to me that parents don't realise that their child understand what is said to them long before they can say the things themselves.
Many children do this either out of insecurity or simply wanting to play.
Although we think everyone knows it might be just as well to spell it out.
Have a bedtime routine which includes a quiet time before getting ready for bed, Read a story and then tell the child it is time to sleep now and not playtime. Then the hard bit for the parents starts. Leave your child to cry for 5 minutes then go in and check they are dry or whatever and tell them it is time to sleep and not to play. Leave them for 10 minutes and do the same, each time adding another 5 mins.
DS took ages to work up to this and I warned him it could take a week to sink in so be prepared and preferably start on a Friday night. He did, left the child for 5 minutes and then eventually went back because there was no sound. Child was in the corner of his cot fast asleep and they never had a problem after that with that child.
They didn't learn, I think number two still goes into his parent's' bed at the age of 5!