I am about to raise my fourth batch of chicks from a broody hen. Here is how to integrate chicks safely into an adult or pullet flock:
Place the chicks in a run inside the pen. This way the pullets can see them, but cannot hurt them or peck them. You can make your own run out of hardware cloth, leave it open at one end, and you can use a bunny hutch with the legs taken off and slide them so that they are connected. Put the food and water inside so that the food won't get wet, and the chicks will not become intimidated by the bigger pullets who will want to scare them away from it.
The pullets will become used to seeing the chicks around, but the pullets are much bigger and stronger than the chicks, who are very fragile. It is not wise to let them out even supervised (I did this once and looked away for a second, and the hens started beating up on a chick. He had a hole in his side. Luckily, he survived) until they are pullets themselves. Even still, begin with very short, supervised outings. When things begin to get a little out of control, put them back and try the next day. After about a week and everything is going very well, allow the now pullets out during the day when you are home. This is for two reasons:
Hawks
Out of control pecking
I once let my bantam and her three babies (my first hatch ever) out while I ran to the store for half an hour. They were right between the age of chicks and pullets. When I came home, I found a pile of feathers, and bantam had led her remaining two to safety in the coop. It took a very long time for me to leave them out unsupervised again, even when I was home. I was always on the watch for hawks. Thankfully, this has been my only predator death of my whole almost two years of having chickens, others have been due to sickness, and one to injury.
If the hens happen to peck a chick, either by accident, or to establish pecking order, and accidentally draws blood, the hens will keep pecking until the chick is dead.
When the pullets seem run when a shadow passes, and can take care of themselves, let them out full time, even when you are not around. Leave the run open, or at least the hutch door open so that they can go in and hide if they need to. If you want to, you can hide a feeder in a clump of grass for them, otherwise, they will simply eat when the others have finished.
If you noticed out of control pecking (drawing blood, the hens/pullets will naturally peck to establish the pecking order, but they shouldn't pursue the matter once the chicks/pullets submit and run away) separate them again for a day or two.
I wish you the best of luck integrating those chicks!