[Fire HD 8 and HD 10 General] DragonFire1024:
I've tried to complete this mission a few times, each ending in disaster...until now.

Summary:

The parental control applications too are restrictive. Amazon made some slight changes to the Android code which gave these applications full, and in my opinion unnecessary control of the tablets and the security options. Disabling or uninstalling them can and will eventually cause you to reflash stock firmware. That's because they are labeled as "device/profile owner applications". Located in /data/system is 'device_owner.xml'. That file configures the apps as device/profile owners and doing anything to stop them results in a disaster, often unfixable without reflashing stock. The idea was to trick FireOS by switching device owner applications right from under it's nose. It's not easy but it is possible.

This will be along guide. one or two of these steps may not even be necessary but I am going to present them as I did them and was successful. Beyond that you are free to remove steps and try the process at your will. However any deviation from the guide can result in damage to your device. Even this guide may result in damage to your device, because not everyone's results will be the same. From here on out you are on your own and I am not responsible for any damage done to your devices. This will take some time, if you do not have it it is recommended you do not continue until you do. It will not be possible to pause for extended periods through this guide. This WILL REQUIRE a factory reset. You must be rooted or have an unlocked tablet to complete the guide. You must also have the Google play store and related services installed as system applications in priv-app folder. For this it's recommended to flash a pico Gapps package in TWRP.

THIS OPERATION WAS PERFORMED ON A DEVICE THAT WAS DEODEXED ON 5.6.1.0 WITH A 5.3.6.0 (LATEST FIREOS) BOOT IMAGE. This will not likely work on a stock ROM that has not been deodexed.

Instructions:

1. You need to download an application that uninstalls system applications. Any one of them will do so long as it works. You will absolutely need to download an application that does this. Also download a root file explorer.

2. Uninstall the application called 'Managed provisioning.' then open a shell using a terminal emulator or ADB. Then type the commands below.

adb shell
su
settings put global device_provisioned 0

3. Go to settings > security > device administrator and make sure Google pay is a device administrator. This is very important and you must not miss this step.

4. Using a root explorer, go to /data/system and copy device_owner.xml to your internal storage. If using your PC please use Notepad++, though it's recommended you complete the following tasks on your device.

4b. Go to system/etc/permissions/ and open handheld_core_hardware.xml in a text editor. Find the line that says: <!-- feature name="android.software.managed_users" --> and replace that line with the one below, save the file and reboot:

<feature name="android.software.managed_users" />

5. Then go to your internal storage and open the file device_owner.xml in a text editor. Replace the lines of text with the text below, making sure the format is NOT changed from the original and save the file:

<profile-owner package="com.google.android.gms" name="Google Play services" userId="0" component="com.google.android.gms/com.google.android.gms.mdm.receivers.MdmDeviceAdminReceiver" />

6. NOTE: this step CANNOT be done in TWRP. Again Using a root explorer, copy and paste device_owner.xml to /data/system, overwriting the current one (remember to back up the original). Set permissions to rw (0600) and reboot. If there are no errors or popups and your home and recents button still work after rebooting, continue to the next step.

7. Reboot to recovery and clear/wipe cache and dalvik and reboot system. When booted, if no pop errors from anything, open the uninstaller app you downloaded and uninstall Parental Controls and Parental monitoring. Again reboot to recovery and wipe cache and dalvik and reboot system.

8. Again if no errors, reboot to recovery again and this time do a factory reset. This step is very important and cannot be missed. It is absolutely required that the data be wiped on the device or the device and system will recognize it is no longer owner of said device and it will start shutting you out of security and other options on the device.

9. When rebooted, Open the shell in terminal emulator again or ADB and type the following command below. A reboot should not be necessary when done:

su
settings put global device_provisioned 1

10. Download an activity launcher (I use activity manager and yes you need to pay a slight fee to use the root part of the launcher, but it's worth it.) Scroll settings and search for 'generic lockscreen set' and long press it to open it with root access. All of your lock screen options should be open and available to you even after removing the parental applications. If they are congratulations you have now remove parental applications and the restrictions that came with them from your device. Go back to /data/system and you will also notice that the device_owner.xml is gone. Killing 2 birds with one stone. If any lockscreen options are blocked then you have failed to properly remove the parental applications and the policies that go along with them.

NOTE: You may need to deactivate 'smart lock' and trusted gms as that may interfere with some lock screen options.

I'll let you guys decide, but I think the device is vastly improved in terms of performance and in Internet connectability after removing these applications.