I don't agree with =X= about the backlight, even though I like it myself on the Palmpilot m500.
If she already are snatching your NOOK I can't see the lack of backlight as a problem.
Can you really get the "new" Kobo for $99? I found it to be very fitting for an 11-year old girl, lightweight and pink

If you want to share the NOOK books without stripping it from its B&N DRM NOOK may be the only choice.
But if she mostly will borrow books on the library I'd recommend the KOBO!