Buying MInecraft Coins for kids from Microsoft account balance?

[Windows Central Question]

I put money into my Microsoft account so when my kids send a request in game to buy coins I can buy it for them. When I get the email and follow the link, I can send them the money but it will not allow me to select my Microsoft account balance as a payment option. What am I doing wrong?
They are playing Windows 10 version of Minecraft and it is up to date. Windows is 1803. Thanks for any help