I got a question a while back from Carie, who wanted to know of a lesser-known disorder she could give a child in her WIP. While the disorder I'm suggesting doesn't have to be just for children, it's definitely one that will give a different spin to a story.
Trichotillomania (which is SO very fun to say, by the way---trick-o-till-o-mania) is an anxiety disorder where the main symptom a person has is pulling at one's own hair persistently. The most common area where children or adults pull hair is of course the scalp, but pulling of hair can happen anywhere there is hair. Eyelashes and eyebrows are really common, as well.
Usually a child will pull their hair or eyelashes when they feel anxious about something just prior to the act. They'll feel tension when trying to stop pulling, as well. After they pull, they'll feel some relief. (So the argument could be made it's a disorder similar to Obsessive-Compulsive Disorder, in that they feel compelled to do it.)
Alopecia can occur, which is loss of hair. It can be extremely noticeable, or small and undetectable....so you can have an adorable looking kid for your WIP without some bald pattern on their head and still have them with a problem.
In preschool age children, this disorder is often considered a childhood habit. My own little girl will often tug at her eyelashes while she's falling asleep, almost like it's comforting to her somehow. She'll pull at them gently, though, not with the intent of pulling them out. Likely, she'll grow out of it. If an adolescent child develops this, then it's more likely that they will have some more severe pathology later, like OCD, so I'd think about what age you want the child to be who might have this disorder and where you'll want to go with it.
A very interesting side note: My cat, Cracker, actually had a version of kitty trichotillomania when we moved him from New Orleans to California. He picked and licked and bit at the underside of his torso and inner legs until he had very little hair (which was extremely noticeable, as he was an all-black cat). The vet called it psychogenic alopecia. Our cat was traumatized by the move, apparently! Like I said, very interesting!
So there you are, Carie. A little-known (or at least little-written about) disorder for the kiddo in your WIP! Any questions are welcome in the comments section, as always.