I can confirm that it acts as a FP for child to ride with family (I used to work at PW). It is a 1 time use - once the date is filled in by the CM you do not get to reuse the card but you can keep it as a souvenir.
Theoretically somebody could amass many of these but it would be a lot of work, they’re not always in stock and as you can tell most people don’t realize they exist!