You'll need to wait until they are at least 8-12 weeks old. Bantams are very small.
Make sure your big pen is large enough the bantams can easily escape the large ones if being picked on. Also make sure there are separate feeders and waterers if needed.
Put your birds in a cage, or divide the coop in half, where the big birds can see the little birds, and leave them for a couple days. Then at night, put the little birds on the roost or whatever, away from the big birds. Make sure you watch closely in the morning, to make sure nothing happens.