They know the # of waterpark tickets based on your room reservation. The kids also usually get little wolf ear headbands on check in.
But don't worry, there are plenty of other add-ons you will be shelling out for. The extra costs include the Magiquest game (the cost of the wands + the cost of game play + special decorations for the wand), the arcade, anything at the spa, souvenirs, food, etc.