My son and his wife bought their 7 yr. old daughter and 4 yr. old son Kindle Fire HD 8.9 tablets for Christmas for books and movies for now. They set the parental control option to not allow them on the internet at all.
Another 12 yr. old granddaughter has the Kindle Fire that you are considering and that I currently own. It, too, has a parental control setting under the general settings. It can be toggled On or Off. ("Restricts purchasing, content types, web browsing and access to other features.") I do not know if once it is set if a child could tamper with it somehow and reset it. In the case of my 12 yr. old granddaughter, my son has set it with an administrator password so that only he can change any settings after signing in. Pretty secure, I'd say.
On another safety note, the two younger grandchildren - different son - have otter box cases on their Kindles, which make the Kindles almost indestructible!! You can view them here:
Amazon.com: OtterBox Impact Case for iPhone 3G/3GS - Black: Cell Phones & Accessories
Hope this helps. They are great tablets for young people to encourage reading.