Kindle FreeTime allows parents to select the content that a kid can see and lock the tablet with a password. Parents can set daily screen time limits, and limit access to content for each child.
It looks like that update is arriving today. It's supposed to be rolled out to your Kindle Fire HD, though I am still running the older firmware. When it is available you should also be able to find it on the Amazon website.
This update might also include away for you to turn off the recommendations, another much desired feature which Amazon promised was coming. But I'm still waiting to get it so I can't say for sure what is included.