Hi Thanh!
Welcome to planDisney! You've asked a fantastic question and I'm thrilled to help a fellow Coloradoan get ready for a trip to Disneyland Resort!
Little Mouseketeers who are 2 years old and under receive complimentary admission to Disneyland Park and Disney California Adventure Park, courtesy of Mickey Mouse himself. So you won't need
theme park tickets or
theme park reservations for your niece since she is 2 years old. You won't need identification to prove her age, so you'll simply inform Cast Members at theme park entrances that she's 2 years old and she'll be able to enjoy the theme parks with you! Keep in mind that you may need identification for her if you plan on flying to California for your vacation, but you won't need that to enjoy the theme parks. So you can store her id securely during your magical time enjoying Disneyland Park and Disney California Adventure Park.
Since there are some attractions that your little princess will be too small for, you'll definitely want to take advantage of
Rider Switch during your visit. It's the perfect way to ensure everyone in your group can enjoy the attractions on their "must-do" list while traveling with a little one.
Feel free to come back to see us if we can help you with additional questions as you continue planning your trip. T-T-F-N, ta ta for now!
~Crystal