The three registry keys to disable GWX and the GWX advert in Windows Update are these...
Then open an elevated command prompt (search for cmd in the start menu, right click and Run as Administrator) and uninstall the following telemetry KBs...
In Control Panel > Windows Update > Change Settings, untick "Give me recommended updates the same way as I receive important updates" as some optional updates have been used to send down unwanted GWX/Telemetry updates.
Also in Control Panel > Windows Update, search for updates, then view the optional ones, then hide three of those KBs above (3022345 shouldn't appear as it's superseded) by right-clicking on them and choosing the hide option.
Now reboot the computer, search for CEIP in the start menu, run it, and change the setting to disable telemetry to MS.
If the C:\$WINDOWS.~BT then your computer is downloading Windows 10 in the background. Search for CleanMgr in the start menu and run it to remove the "Windows Update temporary files" category. Although that may unhide those three KBs above and you many need to rehide them.
Unless MS send a recommended update which adds more GWX or Telemetry stuff to Windows 7/8, your extended family's computers will look after themselves from now on.