I got a Series X preordered. I was unable to secure the Series S for my son but after some thinking I think I'll get him his own series x through the all access option
The trouble is that the game pass will add to your account that signs up to the policy if you go all access. So your kid will be gamepass-less and you'll be sitting on like three years of ultimate. Let me know if you find a way around it because I want to do the same thing
You can set your son's machine as your home Xbox, but turn off auto sign in. He will have access to all your gamepass/live privliges on his machine. Then you can turn on auto sign in on your own machine for your account, but don't set your machine as the home consol. You should be good to go then on both machines with one gamepass subscription.