I would put eggs/rocks/whatever under her for a few days to make her feel like there's something there.  Then at night, trade one chick per egg.  Don't let her see the box of chicks.  Let her hear them, then cover each with your hand and let them go under her and take out an egg.  After you're all done, watch her for a minute just to be sure she's not pecking at them.  I've used this method without the eggs under her before with a silkie, and I had a hard time getting her to accept chicks.  With the dummy eggs, worked like a charm.  And make sure the chicks are day old.  Anything over a week old, and hen's seem to know....