Zanna has the right approach to merging youngsters with older chickens.
I've successfully merged several batches, also, using the "panic room" approach. I just fence off a small corner of the outdoor pen similar to what Zanna describes. After a couple of weeks spending their days in the juvenile pen, I cut small openings into it and let them start exploring the bigger pen. They quickly learn to run back into their little pen when chased or pecked. It's important to have this safe space for their food also, or else they might not get enough to eat since the older chickens tend to chase them away from the feeders.
When they move into the coop, what you do depends on your set-up. If it's not possible to fence off a safe section for them, preferably with its own entrance, then you can try slipping them into the coop to roost ahead of the others. Usually at roosting time, the other chickens are mostly focused on roosting, not picking on babies, and this technique often works quite well.
What I do is on the big moving-in day, I close the coop up after everyone is done laying. Then I move the babies in at least two hours ahead of roosting time. They get to become familiar with their new digs, and will instinctively roost when the light dims. Then at the last possible moment, I let the others into the coop to roost for the night. This is important. If you let them in too soon, there's enough light left for them to contemplate mischief, and the babies might lose their nerve and panic. It can be a tense moment for you until you see they're all safely roosted.
I appear early outside the coop the next morning to supervise the babies' exit. They may need encouragement to come out of the coop. Sometimes the others will chase them out, and your work will have been done for you. But I've found it takes a few weeks of my teaching the babies how to go in and come out of the coop before they get the hang of it.
Other people just wait until the babies reach full size before integrating them, and save themselves a lot of work refereeing. So if you're anxious to get them integrated now, this is what I've found to be successful.
