Pick on the babies

I have 5 silver duckwing batams(1 roo) and 1 4 month old RIR in the big coop. 2 weeks ago when I got my new babies, I had to move my 4 week old babies out into the big coop, b/c i had no where else for them to go. The big chickens killed 1 of them within a few hrs. I ended up putting them in the transport cage with food and water for a day(in the middle of the coop) and trying again the next day. Lost 1 more, i decided it was mainly the roo, so I locked him up for a day. The next day, all was well and the pecking has ceased. My question is, how can i prevent this next time. I suggested to my husband that the bantams need their own coop, he is going to build one as soon as he has time, but time is not something he has a lot of. Any ideas? I have about 80 chicks that are 2 weeks old and they are going to have to come out of the brooder soon. hubby is going to build me a nursery pen inside the barn, but they can only stay there for so long also. Are my big chickens always going to pick on the little ones that bad. ?????