If you cannot assign Gold to additional kids in a group or subgroup, it usually means there are not enough Gold seats allocated to that group or subgroup.
For example, you may still have unused seats available in your overall organization, but if the kids are in a subgroup that only has a limited number of seats assigned, you will not be able to give Gold to additional kids in that subgroup until you update the seat allocation or move kids to a group with available seats.
How to update Gold Seats in a group or subgroup
From your Enterprise account homepage, click 'Go to Groups.'
Alternatively, you can also click 'Manage' in the top menu and select 'Groups.'
Open the group or subgroup you want to update, then click the 'Info' tab.
Under 'Total number of seats,' use the up and down arrows to adjust the number of Gold seats allocated, and click 'Save Changes' to apply the update.
To learn how to assign or remove Gold seats for individual kids, see: How can I give/remove Gold seats?