Hi there Kim, welcome to planDisney!
I always have both a belt bag/fanny pack and a backpack with me during my trips to
Walt Disney World Resort! It's nice to be able to have my wallet, mobile phone, hand sanitizer and other necessities on hand.
The great news is that you're able to take both a belt bag/fanny pack and a backpack with you on most
attractions! Usually, I put my backpack at my feet on the floor of the attraction vehicle, or if I just have a smaller bag, there is usually a pouch you can secure your belongings in while you enjoy the attraction, or just keep it in your lap for the slower moving attractions. There are some exceptions such as
TRON Lightcycle / Run presented by Enterprise in Magic Kingdom Park where you'll leave all of your belongings in a secure locker that you can lock and unlock with the tap of your MagicBand!
There are
locker rental locations in each of the theme parks, and it provides a secure location for you to store your belongings while you enjoy your day. If you have a stroller with you, I've taken my valuables along with me while I ride the attractions, but I've left my backpack with larger items such as jackets in the stroller. You definitely run the risk of it being unattended, but I've never had an issue, and would never leave my wallet or phone unattended.
I hope this information is helpful for your upcoming trip, Kim! If we can help answer any other questions, don't hesitate to
come back and ask!
Have a magical day,
Melissa