When a parent makes an on demand payment that fails it will give an immediate notification saying something like CC transaction failed and also sends an email to the primary email address.

When an Admin makes an on demand payment it will notify the admin immediately with declined message on the site and also send an email to the primary email address of the account holder.

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See Also
How do I allow my parents to make a credit card payment on demand?
How do I refund an Account's/family's on demand payment?
On demand charge & payment for Admins