I'd allow one big chick into the baby area for 15-30 min. Do this daily for awhile and work your way up to a couple of hours, then a few big chicken at a time.
I have 3 week old chicks brooding in the run (sectioned off). They've been out there for 2 weeks now with a heat lamp at night. If I start integrating now (shared time in run etc), how do you control food? Shouldn't chicks be eating chick starter until 10-12 weeks? How do you prevent older hens...