Do you yourself crave crayons? If so, do you actually ingest them?
Like kalisiin said, if a person craves crayons (and they have craved crayons for more than a month) then they are considered to have the pica disorder.
Pica can be caused two two main things: Developmental disability and Mineral deficiency.
Developmental and neurobiological diseases such as OCD (obsessive-compulsive disorder), schizophrenia, Tourette's syndrome and autism can cause pica.
Mineral deficiency, usually of iron or zinc, can cause pica. Pica can also be caused by stress due to traumatic events.
This is just from some research I have done. I have never had a craving for crayons or know anyone that has.