Son purchasing

put him on a parental controlled child account

If you created an adult account you would have to make a new account this is something that has to be done upon creation by the parent

Remove the credit card from the account

Not necessarily. You can purchase them from one of their accounts. The account that purchased the content would then set the other son's console as their Home console. https://beta.support.xbox.com/help/hardware-network/console/my-home-xbox That
will allow the son who did not purchase the games to be able to access the games on their console. The son that did purchase the games can still sign into their console and play the games.

logged in as you go into Settings then Family. "Grant Allowance" to your Son's account. Then your son can purchase.

When you purchase content it is tied to the gamertag that it was purchased on as well as the console it was purchased on. The console license will allow all profiles on that console to access the content.

So if your sons are playing on different consoles, they won't work. If they are playing on the same console then it sounds like the game had previously been purchased on a different console. If that's the case, the son that purchased the game would need
to sign in and complete a license transfer (http://xbox.com/drm) and that should allow your other son to access the game.