I would go for something a bit more advanced like a K10D or a K200D. Finding one for $100 might be a challenge. The budget will need to be upped somewhat but it will be worth it. A *istDL is just a bit basic - six megapixels in an age when any cellphone has double that? Hmmmm.....
A *istDL would also be pretty old and might not have much life left in it.
A K10D or a K200D has all the advanced features that the youngster could grow into, while having all the automation needed to get going without getting into the technicalities.
My teenagers both get along just fine with the K200D that I bought used so that they can stop borrowing my K10D.
Another bonus is that a used K10D or K200D would still get a decent price if nephew decides that photography is not his thing.