In our house we have a kindle fire (mine) and a Google nexus (dh) I would say the kindle fire is limited by its function - which is to encourage you to spend money at Amazon - you can also use it for watching vids on microfilm, audio books through audible. I use it all the time and am happy with it but I wanted a kindle knew what I was getting. Parental controls are easy to set to prevent your child from accessing or buying anything you don't want her to. :-) loves his nexus and it is capable of running kindle app and all the apps on android app store, but is more a computing and gaming tablet.
If all Dd wants is to read books then get a more basic kindle, there are also child friendly tablets by Dd has one called a nabi which is preloaded with games, stories and child friendly apps.
Different horses etc... you have to choose but think what your dd will want from a tablet.
Thank you, I didn't realise you get the the kindle app for the nexus. She does play games on the tablet she has so I know she would enjoy that side of it. I would rather get a decent one now she's older as the joy tab isn't the best tbh