Why can't a child PSN account redeem voucher codes or add funds?

Child accounts (sub-accounts) on PlayStation do not have their own wallet and cannot redeem voucher codes or PSN cards directly. This is by design in PlayStation's Family Management system. All funds must be added by the Family Manager (the adult master account) through their own wallet. The child account can then spend those funds up to the monthly spending limit set by the Family Manager. To add funds: sign into the Family Manager account → Settings → Account Management → Account Information → Wallet → Add Funds.