I have seen this particular question, or a variation of it, show up multiple times in keywords used to find this blog and I figure I might as well answer it.
Many animals will eat kittens if the opportunity presents itself but kittens really do not have animals that generally prey on them. For instance, a raccoon will on occasion eat a kitten (raccoon's are capable of killing things bigger than kittens like small dogs) but generally prefer fruits, insects, small mammals and human left overs. A hawk will on occasion eat a small kitten if it can but generally prefers small mammals less likely to be defended and other birds. Wild dogs will on occasion eat kittens. Human beings will occasionally eat them, an example that I heard about is that in Switzerland farmers would sometimes eat kittens if the cat had an excess number of them using the logic that if they are going to kill them anyways they may as well eat them, I do not know if this practice is still done though.
The animal that kittens are most likely to be killed and eaten by though is other cats. It is not uncommon for cats to eat stillborn kittens and placentas, to regain energy/nutrients from birthing them. Some first time mother cats will eat non-stillborn kittens but it isn't really all that common.
Some reasons mother cats will kill and sometimes eat kittens are:
*can't "switch off" hunting behavior during play and kills the kitten
*thinks kitten is defective
*thinks kittens have poor chance of survival anyway (due to food shortage or external threat)
*Handling by others obscures mother's scent on kittens, so she doesn't recognize them as her own
*Feels odds are insurmountable and can't protect kitten
*To increase the chances of successfully rearing their surviving kittens
*Territory, will on occasion kill rival female cats kittens
*eat it if it died of natural causes to not attract other predators and for food
Male cats will kill kittens:
*smells the scent of a rival tomcat and decides that the kittens have been fathered by the visiting tomcat
*takes over or inherits a territory may be driven to destroy any kittens in order to spread his genetics
*in attempting dominance over a kitten may accidentally kill the kitten
*from being repelled while trying to mate with mother cat might take frustration out on kitten.
Information about Swiss eating kittens comes from this blog Hello Cute Animals and I looked up the Swiss Society for Animal Welfare/ProTier as well.