The solution is what I call a "panic room". It's a protective enclosure for the chicks that the adults can't get into. Make sure little heads can't poke through the mesh and get scalped.
After the chicks have spent a week in this enclosure, and you can bring them back indoors to sleep at night, make little 5 x 7 inch openings in the chick panic room. This will allow the chicks to explore the run and mingle safely with the adults because they will use the panic room to duck back into when they get harassed.
You will need to make sure the chicks have plenty of space in which to evade the big chickens. If they all have very limited space, you may encounter problems and injuries.