Roblox. I had my kids on linux originally, but there's no linux version of Roblox, and the proposed workarounds I found were unreliable, out of date, or more work than I was willing to invest in.
But you are right, a time may come when they will want Windows. Although my wife's computer runs it (needs it as a teacher), so they may use that for Win only stuff. I'll deal with it when the time comes.
I could do a VM with GPU pass-through and Steam client... Although that may also turn out to be more than I'm willing to invest (and essentially you still run Windows). Or maybe I'll dual boot...
I confess I'm not sure; I've trusted Valve with a very specific part of my life (buying games) for nearly 20 years now, and they've held up their end of the bargain the entire time. My trust in them has paid off (so far!)