I recently added a couple of new chicks about that age to an older chicken and it was a little bit brutal at first. Here's what I did:
First try to arrange things so they are as close as possible (and can see each other) without being able to get to each other. This will allow the older ones to get a bit used to them.
Second was to introduce them at night time. If you are not at home during the week, do it on a Friday night so you can keep an eye on them for a couple of days. Get something to squirt the older one with water whn she attacks another one.
Thirdly, if it is just one of the older ones that is bullying really badly, seperate her from the others for a day or two and keep her in the other pen so your chicks are still in the main pen. If they are OK at night then put them all together in the evening.
Also make sure that there is food in several places so that the bullying one can't guard all the food. If they are regularly chased into the hen house then keep some food close to the door so the chicks can get it but escape to safety.
I did try using feather pecking spray (it tastes really horrible, wash your hands after

) but I am not convinced it made that much difference.
I think the main thing thouhg is to have enough faith that they will sort it out eventually. As long as they are not drawing blood and the little ones can get food and water then they probably will sort it out eventually. It took mine about two weeks but they are fine now.