Hey Byoung Joo!
Welcome to planDisney and thank you for bringing your question to us!
This mistake happens quite often. Please do not worry! You will be able to make this change and enjoy your visit. But, you must upgrade the child’s ticket to an adult ticket in person at the
Disneyland Resort Main Entrance
Ticket Booth. A Cast Member will be able to assist you. And, please be prepared to pay the difference in price.
Even though you do not have the correct ticket, you’ll still need to make an advanced
Theme Park Reservation ahead of your visit. You’ll need that reservation in addition to your valid theme park admission to gain entry. You can make reservations up to 180 days in advance. And your reservation should carry over once the ticket upgrade is made. Here’s how to make or modify a Theme Park Reservation, log in to your Disneyland Account and select "My Disneyland." Then, click "Ticket and Reservation Details" and select "Make or Modify Park Reservations." You’ll be able to book a Theme Park Reservation for your desired date.
Enjoy your trip! If you have any more questions, bring them to us here at planDisney!
Tiffanie