Masaki - taken from the Park City web page...
Additional discounts on multi-day tickets are available with lodging booked through Park City Mountain Reservations: 800-222-PARK. Kids six and under ski free (when accompanied by a full paying adult). And don't forget to book your Silver Passport through Mountain Reservations. This gives you a single lift ticket to all three resorts in the Park City Area. The Silver Passport must be booked prior to arrival.