They might try to attack but a mother hen is very, very protective and the chicks stay very close. Introducing new baby chicks that have not been hatched by one of your broody hens is more problematic. They have no protection whatsoever, you need to provide the protection. We recently introduced three new Delaware chicks (a week old) to an existing flock of 4 hens and 1 roo. We kept the chicks in a very large dog kennel facing into the adult bird's coop so everybody could see and hear each other but not make contact. That was over the July 4th weekend. The Delawares still sleep in their separate "bedroom". In the morning I let the adult birds out first. They are very active and hungry and the roo wants his time with the ladies. They are just too aggressive to have the pullets mingling with them. After about an hour when things have calmed down, I let the Delaware pullets out to join them. So far it's worked well. When the chicks are really young they don't know the rules of the flock. But they learn fast, the big girls make sure of that.
I should add that when we first started to let the babies out with the hens they were approx. 4 wks old. We did short outings, 30 - 45 minutes at a time while we were right there. Then we'd do an hour or two. After a few weeks we left them out all together for the day. But at night, the babies go to their kennel and the hens go to their coop.