The best thing to do if you have the facilities is wait until they are 4 months old, or about the same size as the adults.
If you are not set up for that, then it is best to wait as long as possible. You can always set up a different daytime run for the chicks and put them in the coop at night in a dog travel crate or something (or garage). Make sure you give them protection from hot sun and rain, and windbreaks also for daytime.