My home is more of a democracy than a dictatorship. I could of course forbid them from playing, impose whatever sanctions, But they have fun doing it and they have a bunch of real life friends that join them.
It's been a little while since I tried last thing I tried was the bottles method and it was not going well.
I'd be willing to give it another shot, But if I do find something else that works I've got to make sure that continues to work. If somebody changes something and They miss out on a Halloween or a Christmas live event I'll never hear the end of it.
You can definitely give a VM enough power through KVM and qemu to run Roblox. You can even try your hand at single gpu passthrough if it's worth that much time having youe kids switch to Linux. Not putting a value statement there, I think it is but that's besides the point.
You can also give quite a bit of power to virtualbox with 3d and hardware acceleration.
Could try finding them a new game to play on the new system. I've never played Roblox myself so don't have any suggestions, but I'm sure there's something that will scratch that itch for your kids.