Welcome to planDisney, Mickey! We are glad you brought your question to us.
It sounds like you have been keeping up with Disney Parks news! You are correct that there was recently a change regarding
Disney Park Pass theme park reservations and
1-day Walt Disney World theme park tickets. At this time, all Guests ages 3 and up need both an admission ticket or pass AND a Disney Park Pass reservation in order to enter a theme park. Guests purchasing 1-day tickets no longer need to make their Disney Park Pass reservations separately after purchasing tickets. When purchasing 1-day tickets, Guests select which park they would like to visit, and a theme park reservation is automatically made for them at the time of purchase. Guests purchasing multi-day tickets will still need to make a Disney Park Pass reservation for each day of theme park admission.
If Disney Park Pass reservations are "sold-out" for a specific park, tickets will not be available for purchase online or at the gate. On some of my recent visits, I saw signs posted at parking lot entrances and at ticket windows indicating that park reservations and tickets were no longer available. I wouldn't want you to show up at the gate and be disappointed. However, availability can fluctuate as other Guests' plans change. Just today, I did see online ticket availability briefly come and go for December 30 and 31, so you may wish to keep checking.
If you are unable to purchase theme park tickets, perhaps you might consider bringing your niece to
Disney's Blizzard Beach Water Park -- no reservations are necessary.
Mickey, if you have any additional questions, please feel free to visit us here at planDisney again. We are always happy to help.
Anne Marie