mulitple players

It is recommended to create a separate (free) account for each of your kids.

Also do NOT use your own adult parent account for your kids.

You should create a separate account for each of your kids with the proper birthdate entered in the account (so the Xbox service can enable online and privacy settings specifically for children), and if not already done so, create an adult account for yourself.

Then you can
add your children to your parent account, and
control what the kids are allowed to do (or not allowed to do) on the console.

With each kid having their own account, they can later go separate ways when they get older, and take their own gaming progress with them.

With one shared account for both your kids, it's like giving one kid the crossword puzzle that the other kid has already solved and filled out.

Thanks to the Xbox One's "Home Console" feature, each kid can play the other kid's games on their own account. So it's sufficient, if one kid buys a game or a Xbox Live Gold membership for online multiplayer.

The other kid can then benefit from the Gold membership, as long as the owner account of the Gold membership enabled the console where both kids play on as their "Home Console" (on the console under Settings->Personalization).

If you have further questions about "best practices" for child accounts, please feel free to ask here. We're here to help!

I guess it would depend if you want to each track your own achievements etc / use parental controls and restrict access for some users.

Can share Gold across family and each have own GT, or most games either don't need a second GT, or have a guest / +1 option (shows as GT(1))

Yes you can. Using the SECOND controller press the XBOX button in the centre and then sign in the second profile. For reference, not every game will allow you to have a second "guest" into the online multiplayer if they are a SILVER member (Free Member).

To elaborate, most of the time both player or Gamer Tags would require to have an Xbox Live Subscription in order to both be online(Live) and play a multiplayer game.

Some games such as Halo and COD will allow one Gamer Tag that is Gold Status bring on a Silver Gt as his guest and play on line.

Hope that helps.

A good reason to have mulitple saves.