answer:They stay warm and cozy by being properly dressed (either a onesie, sleeper, or both depending on the temperature) and swaddled (while little enough to stay swaddled). At least, that’s what the newest recommendations are. No blankets, bumpers, stuffed animals, or anything else in the crib other than the fitted sheet they are sleeping on. That being said, my two month old has a breathable bumper on his crib and I put a receiving blanket over his legs to be sure he stays warm because he won’t stay swaddled and he sweats if I have him in a sleeper at night.