S.P.
Have you discussed this with her pediatrician?
Have her tested regarding levels of various metals in her blood.
Lead? Iron?
Don't let her have access to paper unless you are supervising.
About crayons . . . I used to chew on crayons but not eat entire crayons.
Just would nibble on the ends. Some colors tasted better than others.
Consider offering her raw carrots (or jicama, broccoli, etc.)
when she wants to chew on something.
Also, perhaps, ice chips.
Good luck.