How can my kids still use my Xbox Live Gold sub on our old Xbox One while I'm signed into...

That has already been done. Users on that console can't play games that require Live access without me being signed in. In this case Roblox. My understanding is that Live should work on that Home Xbox even with me signed out once it's designated the Home Xbox.

Try holding down the power button on the console until it turns off, unplug it, plug it back in, then turn it back on.

The Home setting has not been discontinued. You would set the old One as your Home in order to share you Gold/games. https://support.xbox.com/en-US/help/hardware-network/console/my-home-xbox

I think you might be misunderstood. So you'll be happy to know that it is in fact really cool. If you have a Gold Sub and you have an Xbox One console that's your "home console" anyone can sign into that console and use Gold Features with their own free/silver
gamertags. If you have an Xbox One but not Gold and I come over and sign in, you can use your Xbox as a Gold Sub Xbox until I sign out and go home. Additionally, you can access all of my games while I'm there signed in and use it with your gamertag.

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news.xbox.com/.../xbox-one-digital-games-and-live-gold

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www.ign.com/.../this-is-how-xbox-ones-game-and-gold-sharing-works

We run 2 xboxes all the time, basically we have our front room xbox set as the home xbox for the account with gold. Any other child accounts and other adult silver accounts can play on this xbox with out the gold account being signed in. The backroom xbox
is setup for the gold account to log into, this xbox is not setup for any account as home. So the only account that can really use it is the one that has gold which is my account. So I can play in the backroom while the kids play in the front. So what we do
if both kids want to play on separate xboxes? I sign my gold account into the backroom and then allow one of the kids to also sign in. As long as the gold account is logged into the backroom both kids can play on either xbox. As for content it needs to be
bought and downloaded by the account that has gold for everyone to have access to it on the home xbox. Hope that kinda gives you a picture of how it works.