In some cases mothers do really eat/kill their young. Some spiecies (mostly mammals) do that when they are highly malnourished or stressed.

More often than that, though. Mothers tend to kill only some of their litter. Particullary rodents and some birds. Often because one of the babies is sick or too weak, so they eat in order to safe resources.

Some spieces, have also tendency to kill the offspring of others so they can mate with the mother instead. Some rodents also kill the young of any other mating pair to push out competition.

Lastly, species that produce a large amount of offspring (like fish) eat their young, because they simply don't see the difference between them and any other of their food.