Create a windows to go install on a USB to NVMe caddy, use something like nano11 and set the BIOS to boot off the NVMe. Whenever the kiddo wants to play fortnite, just plug the device and reboot
This is to avoid windows overwriting your EFI partition and nuking your bootloader at random times, the reason I stopped dual booting after win 8.1
Sorry, I jumped ship before that feature was a thing. I think it's exclusive to "pro" win10 but I remember a relative using it. Windows 2 go or something like that was the name.
Anyway, happy new year, and hope you can figure it out!