Hey there, Juan!
Welcome to planDisney and thanks for stopping by. I see that you have an upcoming visit to the Disneyland Resort! I'm certain you want to make sure that you're all set for a smooth trip, so let's get to answering your question.
A child's
Theme Park Ticket is meant to be used by a Guest ages 3-9. Any Guest that is aged 10+ must use an adult ticket. In response to your question, it will be fine for a child to use an adult ticket (even a discounted one) but an adult may not use a child's ticket. You won't be able to recover the difference in cost between the two tickets.
Since your visit is taking place in just a few short days, are you preparing your Pixar and Star Wars knowledge? You'll be able to catch both
Season of the Force and
Pixar Fest as both festivals are happening at the Resort! There are even Foodie Guides for both events that you can study ahead of time to know if there are any special dishes that you'd like!
Juan, I hope that this was useful for you as you prepare to visit the Happiest Place on Earth! Please return again if you have more questions because we'd love to hear from you. You're always welcome at planDisney!
Wishing You a Great, Big, Beautiful Tomorrow!
-Al