All the ones I have hatched under broody hens, have turned their noses up at chick crumbs. My friend gave me some she had left, and when mine hatched last week, I offered them some and again they left it. Seem to be eating mixed corn with mother hen, but she does dig in the pen, making an awful mess, finding them grubs and insects.
We had a hen hatch chicks away from home on a railway embankment, and they survived, presumably on what mum dug for. A broody hen is amazing to watch when she is digging for food for her brood .....she never stops!!