I wanted to add something. The big ones ARE taking care of the very tiny bantam. Yesterday I had them all outside. It was warm so they were out even when the sun was going down, huddled in a bunch with the littlest bantam in the middle of the pile. One of my australorps sits on her, looking just like a mama. She was very warm and cozy when I picked her up to return her to the brooder. In fact they were all so warm, I considered letting them sleep outside (2 1/2 - 4 week old chicks) but decided to return them to the brooder anyway.
The two bantams were introduced to the larger group of babies when the older chicks were about 2 weeks old. One of the bantams was about a week old, and the littlest was only a couple of days. They have never picked on her. The older bantam is a roo and tries to dominate the bigger ones, kind of funny really because he is quite a bit smaller. There isn't any fighting whatsoever.