On the Sign Up view, you'll notice a pre-selected option for "I'm older than 13". Unchecking this will change the "Sign Up" button to "Provide Billing Information" which leads to a credit or debit card entry view. 

To comply with child privacy laws defined by COPPA, anyone who is under the age of 13 must confirm parental consent with a valid credit or debit card.

When a credit or debit card is accepted, we'll need to charge a $1 purchase which will immediately get refunded.