Game pass ultimate on child account but live on adult account

They won't automatically be cancelled since they were purchased on a different account. If you're not going to be using it or don't want to keep it, you can cancel it. You can also cancel it and just set the console as the Home console for your son's account
so you can share the subscription from his account. https://support.xbox.com/en-US/help/hardware-network/console/my-home-xbox

That’s what I thought, just checked my account and it looks like everything has already been cancelled, thanks for confirming!

We seem to be having the same problem. The child account has games downloaded which we cannot now play despite having just added 12 mnths ultimate to the main account. Does the 12 month ultimate apply to both adult and child account?

Hello, first it's a pleasure to be able to help you today.

My name is welcome to the community.

Hello Laura, this message appears because your child's account is a child and it is not enable for purchases, in which case you can put your child's account in a Family and enable the purchase of the subscription for their account.

The other way to uprgrade from Live Gold to Game Pass Ultimate would be to buy a 25-digit Game Pass Utimate card and redeem it in your child's account, so the time he has of Gold will convert to Ultimate.

Unfortunately Xbox live discontinued their family membership to pursue game pass, however I do recommend that you have the 2 adult accounts with the Xbox live membership and share with the child accounts via home share feature on consoles