Hey Ramon!
Welcome to planDisney and thank you for bringing your question to us!
The mistake you’re describing actually happens quite often. Please don't worry! You will be able to make this upgrade and enjoy your visit. But, to upgrade a child’s ticket to an adult ticket, you will have to make this change in person at the
Disneyland Resort Main Entrance
Ticket Booth. A Cast Member will be happy to assist you. Please be prepared to pay the price difference in full.
Also, you’ll need to make an advanced
Theme Park Reservation ahead of your visit. That's because you’ll need that reservation in addition to your valid theme park admission in order to gain entry. You can make reservations up to 120 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