i too am going through this same thing, i had free ranged them for about a week,, forcing them to be close to one another,, the older ones would peck at the little ones and they would run and hide. the older ones would eventually leave them alone as long as they were not to close. after about a week i would put the little ones in the coop watching closely for 30 minutes or so and eventually walk away leaving the door opem. the older ones would run out and start eating the grass, any way,, its been almost a week sice they have been together in the coop all night and all day unless they were out free ranging. the little ones would stay, eat and drink. the older ones are leaving them alone now and for the first time since the little ones came out with the older ones as one flock.. PROGRESS!!!!!\\\\
so far so good, the little still watch each others backs,
good luck