Putting them under her at night is the best plan. I had a dozen eggs under my hen and she was able to cover them all comfortably. If they had all hatched, I guess that would have meant 12 chicks were a good fit too. How many chicks are you getting?I am getting some chicks in the mail in the morning and am going to try and put them under my broody hen who has been sitting on infertile eggs for over a month. I hope she accepts them. She is a big hen, so how many chicks do you think I can put under her? Also do you think the chicks will be able to get out of her nest box (front opening about 3 inches high on the bottom) and get food and water. I am going to make sure they are eating and drinking after they arrive and then put them under her at night.
If you are concerned they won't be able to get in and out, perhaps you can put something there for them to use as a stair? When my broody's chicks first hatched, they couldn't get in and out of the broody coop. I put a brick on either side of the door opening and they used that as a stair to jump in and out.