Welcome to planDisney, Edwin!
I'm sorry to hear that your friend can't make it to
The Most Magical Place on Earth. However, how wonderful that this opens up the opportunity for you to visit instead!
Whilst
Walt Disney World Resort tickets are priced differently for adults and children (ages 3–9), a child can use an adult ticket without any issue. I've actually done this for my own children. The system won't flag or prevent entry, so your 5-year-old will be able to enter the parks and enjoy every bit of the magic. The reverse, however, isn't true—adults can't use a child's ticket.
Once those tickets are reassigned and linked to your
My Disney Experience account, you can make any park reservations (if needed) and start to prepare for your trip. I recommend purchasing
Lightning Lane Single and Multi Pass to maximize your day in the park.
I hope that's clarifying, Edwin. If you have any more questions, you've got a friend in me. Have a wonderful trip.
Mike