How to set up a family account with a child that doesn't live with me

Ok community. Let's see if anyone has run this gauntlet.

Here are the factors:

• I want to take on the subscription for Xbox Live for my kiddo (10 years old.)
  
• They live with their other parent.
  
• Their account and game data has been set up under their other parent's email/credentials.
  
• I set up my payment/wallet on their console, but would like to manage all this online since I don't have access to that house/console much.
  
• I used to have a Live Gold but don't anymore. I could renew it if I can make his a child account but don't want to pay for two since it's all for him anyway.
  

Anyone know if Microsoft will accommodate this? Can I just open a subscription and move his account (that's under the other parent) to mine as a child account?

:)