Although there are many stories designed to scare children into behaving, this one is probably the most disturbing. A lot of other stories are based on some thing or creature that actually exists but one can only hope that the same can't be said about the food monsters. If a child is being a picky eater their parents will tell them about the food monsters. If the child is refusing to eat just one specific thing they will often name it that, for example the "Broccoli Monster" or the "Meat Monster" (yes, that one is a bit unfortunate and the parents should really not have been specific about that one).
The story goes that if the child won't eat everything on their plate, or at least try a bite of everything, the leftovers will turn into monsters during the night. These monsters are described differently depending on what kind of food it is but they always have large mouths with sharp fangs. Once the food has transformed into these monsters they will sneak into the child's room and devour them whole. But if the child at least tries everything then this either won't happen at all, or the monsters will notice that the child has some pieces of them in the stomach and they don't want to eat something that's like them. Then they'll return to being just food again and the child will be safe.
Variations & Mutation
There is a milder variant where if the child doesn't eat their vegetables they will turn into one.
This old story has lost the popularity it once had. More modern parents aren't as eager to scare their children into obeying and some even consider it abuse. There are still some parents who use it to scare their children into eating what they are given.
If you refuse to eat everything on your plate you are in for a terrible fate
For at night when you go to sleep, the food will grow huge and monstrous and to your room it will creep
Opening their newly grown mouths to bare their fangs, they are about to still their hunger pangs
Slobbering, dripping and oozing, you are the food they are choosing
So be careful little one with what you leave behind, for the food monsters will not be as kind
Thistle Ambertrapper in "Fairy Tale Poems"