Hi, Quoc-Tuan!
Thank you so much for stopping by!
I think there's a solution for your niece, but it would require booking a
hotel room inside the Disneyland Resort. The Disneyland Resort has a separate park reservations inventory for Guests who have room reservations, and that's a fantastic alternative for visitors who need last-minute reservations.
You can check this separate park reservations inventory in
this calendar, and I advise you to move fast if you find availability. After booking her room, she has to
link the reservation to her Disneyland account and then
book her park reservations. She should be good to go then!
I hope this helps you and your niece have a lot of fun together at Disney California Adventure Park, Quoc-Tuan!
May the magic be with you,
Julie