Cats also kill their babies if they find that they can not keep them. They know when they are physically at their max and if she is unable to feed all, and sees some are starving, nature takes it course.
If she is in too cramped of a place, that might be an issue as well.
Eating the baby, if she is completely reliant on YOU for food could mean she is getting a horrible diet. Give her something organic and not based on corn. Dr Weil has a really good pretty cheap food for cats that has a lot of real animal protein in it. Mom needs iron, blood has iron. She may be eating the dead babies for a source of vitamins.
She may be eating them because that is how they are disposed of too ? Who knows..
Give her better food so she can produce quality milk.
Give her a bigger bed if need be as well
other then that -leave her alone- . Cats survived THOUSANDS of years and MILLIONS of kittens with out us stepping in.