There is a noticeable difference between the 6th gen nano and previous generation. Look at the 6th gen's profile, and you'll see that the 6th gen's glass sits outside/above the aluminum shell rather than laying flush with the case on previous gens. This design would make the type of damage the OP experienced rather easy to reproduce; the glass takes the brunt of the impact rather than sharing it with the aluminum.
Frankly, that worries me, especially since my wife is considering getting my child a new nano as an upgrade to a Sansa Shaker (which I can easily recommend for young children-- it can take a beating, and has a built-in speaker for use with or without headphones). I'm trying to talk her into getting a refurb 5th gen.