From your Enterprise account homepage, click on Go to Groups and select the group where you want to update your Gold Seats:
If you want to remove a Gold membership, click on the badge icon with the red x next to the Kid's username:
If you want to give a Gold membership, click on the badge icon with the green + next to the Kid's username:
If your kid has Gold but you don't see a + sign, it means they obtained it a different way (they bought it themselves, for example)