If your kid is already setup into your Family Sharing, the Wallet app doesn't let you invite that kid. And you cannot change the birthdate without uploading an ID.
The alternative is creating a new user with a falsified birthday. But that seems like an awful lot of trouble to claim $10.
This’ll also establish a credit history for your child if they don’t have one yet and keeping it around will lower their credit utilization even if they never end up using it. And you get 9 bucks and a pack of gum. Sounds like win-win to me.
Only if you supply their SSN to Goldman. If a parent doesn't want to supply their child's SSN or any other person you share the card with you don't have to.