First of all, you should leave the kitten(s) alone.  And watch from a distance for the mother to return.  Often times she off hunting for food, therefore she will return to take care of her kittens.  However, it is very important that you keep your distance, because if she is feral she will not come back to her kittens if a human is present.  If you do not see the mother or evidence that she been back after 24 hours, or if you know for a fact that something has happened to her?  It is best to remove the kittens from harm.  And contact a rescue.