Ok, this has been an incredibly frustrating experience. I had long ago purchased Minecraft through Mojang, now it's part of Microsoft. I migrated my original game to MS and my kids played it for a while, and then if vanished into the aether. It's no longer accessible through my Microsoft account or Mojang. After communicating with support, I finally gave up and bought Minecraft again, the Windows 10 Java edition, through the Microsoft store - on the premise that online answers here and elsewhere seem to indicate that I can share my copy with my kids so long as they are part of my family group.