The younger chickens have been in an enclosed part of the run for about a month now. They are separated by wire so they have been able to see each other. They just recently started free renting together and every day gets a little better. I think, like you said, they just need more time. Thank you for your suggestions!
You're welcome!
We let ours mix with the adults during supervised evenings in the spring. They are all (chicks and hens) so interested in all the "new" things to explore after being in their run all day that things are pretty relaxed during free-range time. It ALSO gives everyone a chance to mix without the wire between them, giving the chicks a chance to slowly work their way into the fold...