My bantam game hen just hatched 2 appleyard ducklings and she is looking after them quite well. I dont think the size of the ducklings have bothered her but she may get a bit confused when they get bigger than she is, but hopefully they will be more independent by then. I had a large hen hatch and look after 4 aylesbury ducklings and at 3-4 weeks old were almost as big as she was. She would have looked after them for weeks more but they didn't need to be kept warm by her at this stage, because of the summer weather, so we put them in with the other ducks and they are growing like mad and are happy. I thought it about time they realized they were ducks not chucks. Your bantams will probably be fine and happy they get any babies, if broody enough. Mine were.