Check out this article to learn how to change your Apple subscription: Apple Membership Changes/Cancellations
To add an additional child to your membership, you will need to purchase a new membership plan that accommodates the total number of children.
For example, if you originally purchased a 1 Kid, 1 Year membership plan and now wish to add another child, you will need to purchase a new 2 Kid, 1 Year membership plan.
What happens to my old membership?
When a new purchase is made, our billing system will automatically apply your remaining balance to the new purchase, replacing the previous membership and timeframe.
Shall you have difficulty changing your plan, please contact Apple support, as we're given very limited access:
Apple support phone 1-800–692–7753
Apple Support webpage