What I did in the past (and did work btw lol) was I got a smaller guinea pig sized cage and put the new babies in it (food and water obviously) and set that cage into the existing hens coop. This was only for a short time period each day. And I gradually increased the time "exposure" to the adults. Slowly but surely the adults got used to the newbies. However, they were of adolescent stage by the time I felt they were ready to be introduced without need of a seperate cage.
Im sure there are other ways that work good too. This is just the method that I tried and worked for me. Thankfully!