I think the Navi is too kiddy for her--a lot of the ads show much-younger kids. in a couple years she'll be in middle school, and by then she'll definitely outgrow it. kids grow up real fast between like ages 9-11 lol, my little sister sprouted from a child to a tween in no time. I mean at 9, she already has the fine motor dexterity not to require a giant rubber grip and large buttons, so she's physically capable of using the N7 just fine.
With a secure case, child-proof app launchers and whatnot, you have the option of downgrading the N7 to a very child-friendly device. but you can also upgrade to a tween-friendly device. So better specs at the same price, and the ability to grow with her--I think the N7 would be better, but that completely depends on how long you think she'll use it before she moves on to another device.
also to keep in mind--your decision also depends on whether or not you'd be ok with her using social media (fb) in middle school; because if she still uses this when she's 12 and you don't have the child launcher, I don't know if you'd want her secretly facebooking on it like my sis did with her ipod.