The best way, and only way that has ever worked for me in 40+ years, is to put the babies in a separate space inside the coop that allows the older hens to see them, but not be able to get to them. Also this gives the younger ones an escape if they need it. After about a week or so let them out together. Just open up the door to the rest of the coop and allow the younger ones to mingle out as they feel comfortable. It may take a couple days before they go outside, but that's alright. By this time the older ones know that they are not intruders and shouldn't be so aggressive. There will be some stress while they work out the pecking order, but they shouldn't be trying to kill them any more. Also the younger ones will stick to themselves for a while but in time they will integrate and learn to get along. However, if your coop is too small this won't work because there has to be a place for the younger ones to go if they need to get away from the older hens.