Over protective momma duck!

I took down the barrier that was seperating momma duck and her babies from the others. I left one chicken in with her and the others went to a different coop. There are 2 heating lamps on both ends of this big coop. So yesterday i figured i would move my 4 week old chicks outside to that coop in my barn. I figured momma duck would be fine and not see 3 little chicks as a threat. I figured my other chicken that was in there would take to them and share the other heating lamp and nesting area with them. This is a fairly big coop, its probably about 12 x 8..at least.. I put them in there and my chicken was ok with the baby chicks but momma duck attacked one of the baby chicks. I had to save my poor little chick from her, she was really mean..It was not easy to stop momma duck from attacking. How can I get her to see they arent a threat. I really want to move my chicks out, I have more chicks coming tomorrow that need to take over the indoor brooder. All my older hens and ducks are in a bigger coop all together and i dont want to move momma duck with them either. Maybe I will have to just build another brooder today...