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.
Wouldn't openbsd absolutely take the cake then? Freaking OS is more secure than a vault