Xbox one purchase of the Xbox 360 game for gold.

A future update (March's, probably) will allow you to see the Xbox 360 Games with Gold in the Xbox One's 'Gold Hub', and will allow you to purchase the game from the Xbox One itself, so going forward you should not run into this. Until then, all Xbox 360
games need to be "purchased" on Xbox.com or Xbox 360, as was the case here.

Hi

Unfortunately Xbox 360 game purchases can't be completed on an Xbox One console or at Xbox.com without having a payment method such as a debit/credit card registered to your account. One you have done this it should allow you to complete the Xbox 360 games
with gold purchase without charge.

If you try to complete this purchase on an Xbox 360 console you will not need to register another payment method.

Please note not all Xbox Live Gold subscriptions (typically trial subscriptions) include Xbox 360 games with gold games.

If you do get an Xbox Live Gold membership for your Xbox 360 don't forget each month to redeem the Games WIth Gold games for both the Xbox 360 and Xbox One, this way if you do purchase an Xbox One at a later stage as long as you still have an active Gold
subscription the Xbox One games will be available for you to download and play in addition to the Xbox 360 games.

To claim 360 games with gold for xbox one you need to have linked with the account a valid method of payment, credit card. In this case a purchase of 0,00$ is registered while claiming the game.

Since the account is that of a minor and not allowed to purchase online, it cannot claim the 360 games with gold for xbox one.

Xbox one or xbox 360 games with gold

If xbox one you must reduce the content settings filter

Also if its xbox 360 you need to do it on an xbox 360 or use your account

Also home gold doesn't extend to the website only the master account can make gold purchases on the website