M.M.
Start while they are young. Put the child to bed and allow them to fall asleep on their own. Crying is hard to take, ungodly hard, but children learn that nighttime is sleep time. Place them in bed, allow them to cry for ten minutes, then go and check on them. Do not talk to them, do not pick them up, just retuck and be on your way. Do this while they are young and you won't have a problem when they are older.