Foolishly, I trusted Microsoft's assurances that the girls' computer and apps were Windows 10 compatible. Foolishly, I believed that things could be rolled back the same day that I'd run the Windows 10 upgrade.
Lies. All lies.
I tried doing a partial restore from the Acer hidden partition. That reinstalled Windows 7 partially, but the install couldn't finish for some unspecified reason. I finally gave up and wiped the old installation out completely, losing all of the Minecraft Worlds on that machine, but I did manage to get Windows 7 to boot again before going to bed some time after 3 AM. I set the machine to download updates, of course.
A shame I didn't think to turn off the sleep function. A shame that the sleep function will shut the machine down in the middle of downloading updates too. Who designed this mess?
Oh, right. Microsoft.
I turned off the sleep function in the morning and managed (finally!) to get the updates downloaded and installed. And then I had to reinstall the critical software which was -- happily! -- not much.