We got a Fire for DS5 last Xmas: he's 10 now. We tried the parental controls, to limit his time playing games, and to keep him offline when unsupervised. While you can set up time limits for different activities (games vs reading), and cutoff times, it was unreliable. Sometimes it would cut DS5 off after 10 minutes when he clearly had an hour left, and I could not find out why it didn't follow my rules.
The controls work thru setting up different users on the device: a parent user to set controls, and the limited child user. Believe me, the kids will at some point get access as the parent user, and all controls are essentially off. Access to online is completely blocked or granted, not judiciously limited, for kid accounts, which was much too limiting. The device is designed to keep kids safe from the big bad internet thru their FreeTime service - pay the monthly fee for access to a safe playground for the kids. Opt out of the monthly service, very little to do other than read books you load up for the kids.
I was disappointed with the way the parental controls worked - unnecessarily complex to access and set up and understand (for parents), too limiting for my DS5's age (probably better for the preschool age set), and definitely only useful with their paid service. And flakey.
I first got a keyboard e-ink kindle, and love it for reading. I have a Fire, but use it more for internet access than reading these days. The glare on the screen is annoying, the e-ink screen is much better on the eyes, and font controls are better, too. Visibility in bright light is a problem with the Fire, too.