I have two adult, well established hens, and my new chicks are 6 weeks old. When they were 4 weeks old, I introduced all 26 pullets to the 2 old hens. First I cut down a bunch of big tree branches and put in the pen. The chicks could dash under and through the branches but the big hens could not get through the "forrest" of branches and leaves. It worked AWESOME! It took the big hens about 30 minutes to figure out that if they try to chase a chick, the chick runs into the cover, and the hen gets poked with a protruduing stick coming off a tree branch. It was sort of a self training system. Go after chick = get poked with a tree branch.
Every morning I would let the two big hens out of the pen to free range, so they were away from the chicks, but in the evening they would go back in the pen. After a few days of this, I could safely leave the hens and chicks together 24/7 in the pen and have no trouble. It's been about 2 weeks and they now all eat together, roost together, drink together, they have no problems.
My old hen is going on 7 years old so she had VERY little inclination to buck the system. The other hen is about 1 year old and quite a bit more energetic and feisty, but the tree branches poking her in the breast meat when she tried to dive in after a chick put the kaibosh on her in a hurry!
And the chicks absolutely LOVE having all those sticks and shrubbery to climb around, through, and under. They are in chicken heaven!
I used white pine and apple. I think cedar is poision so you'd have to be careful about that.
I wanted to try this tree branch/shrubbery approach because I was afraid if the chicks had a place to retreat and hide to, they would just hide and retreat and avoid the big hens completely. With having all the shrubbery, they felt safe to venture over right next to the big hens, knowing the big ones couldn't grab them. They would come flying out of the leaves and cruise around a lap and then crash land back in the branches, and do it all over again 3 minutes later. It seemed to give them a lot of confidence and security. The big hens seemed a little confused by all these chicks rustling through the brush. It really worked out great and I would definitely use this system again if I needed to.