First things first:
with a warning attached...this place is waaaaay too addictive!
Now, on to your problem. While I've not had this problem myself, having always had a small to moderate sized flock, the rules of introduction still apply regardless of numbers or age.
You need to be able to keep both parties (older mature hen and babies in this case) separate, yet within view of each other. They need to be able to interact, without direct contact, with each other. An ideal set up would include a divided coop whereby the mature hen has her sheltered area and the babies have theirs. Each separated from the other by a chicken wire divide.
Introduction of the babies to this environment first would be preferable, as this would establish ownership of the territory by the younger birds first. Let them be in there for at least a week, prior to introduction of the older hen. Then introduce Henny. Sit back and observe the reactions of both sides. If Henny shows undue aggression, i.e. throwing herself at the barrier, seeking a way around the barrier, pecking through the chicken wire any time one of the chicks approaches to near, you may have an uphill climb on your hands. But, give it a few days. She may relax in time and come to accept the youngsters. In this case, slow, gradual acceptance of the youngsters could be achieved.
On the other hand, if she shows a genuine non-hostile interest in the chicks, she may have a strong mothering instinct. If this is the case, I would recommend introducing just one chick at a time into her side of the enclosure. Again, sit back and observe their reactions. Watch for any hostile body language on either side. Even young chicks can be aggressive and intimidating.
If Henny displays a nurturing attitude, I would still give it a few days before access to the younger birds is granted.
With any age group of chickens, integration of new members to any flock should be done on a slow and gradual basis. The more time that birds have to become accustomed to new introductions, the better. Safeguarding the health and well being of each prospective new flock member is your responsibility. Be prepared though. Some integrations never work out due to the personalities involved. But, since Henny has been with other chickens during your rehabilitation, it's very likely that she will accept new chicks as just another flock to belong to. Just give her time to adjust to the notion.
Good luck and keep us posted on what your decision is and how things work out for you and Henny both! You'll always find folks here who share your love for chickens here. Never hesitate to ask questions, or contribute your thoughts. It's how we all learn to make the world our little feathered friends live in to be a better one. Again, Welcome to BYC!!!
Amy
