chickenhappy1233 is right, it depends on what kind of birds you have. Certain breeds will think they are cute and love them, others will want to teach them about the "peaking order". Your mother hen will protect them with her life. Regardless, it wouldn't be a bad idea to watch them and make sure they are safe.
Broody hens were hatching and raising chicks with the flock long before humans domesticated them and for a long time after they were domesticated. Last I heard chickens were not extinct so obviously every hen does not automatically attack a baby chick.