Hello Andrea, and welcome to the Disney Parks Moms Panel.
As a frequent guest to theme parks at the Walt Disney World Resort, I always have a backpack or sling bag of some type. Depending on the attraction, you may be able to wear your Kavu on rides. You will probably not need to rent a locker to store your bag for the day.
There are some rides and attractions that will require you to remove your bag while riding. For your own personal safety, there may be a lap bar, seat belt or other restraint that would require you to store your bag in a designated pouch or area during the ride.
Star Tours - The Adventure Continues,
Mission: SPACE and
Avatar Flight of Passage are a few thrill rides that come to mind. A Cast Member will advise you if you need to store your bag while on a ride.
Keep in mind that all bags are subject to search, by
Walt Disney World Security, before entering the theme parks.
See ya real soon,
Andrea