Hello Phil! Thanks for bringing this great question to planDisney.
Most backpacks are allowed to be brought into the Disneyland Resort, but there are a
few types of backpacks that are not permitted. Backpacks that have wheels or backpacks or similar bags that are larger than 24" long x 15" wide x 18" high will not be allowed through the security checkpoint. I recommend double checking the size of your backpack before bringing it on your trip to make sure it fits within the size requirements.
I have not had an issue bringing my backpack or a small crossbody or fanny pack on the attractions throughout both Disneyland Park or Disney California Adventure Park. At times, my backpack was a bit too large to fit underneath my seat on
Soarin' Around the World in Disney California Adventure Park, but a Cast Member helped me put it to the side during the attraction, and I easily grabbed it on the way out.
There are also
lockers for rent inside both theme parks as well as in the Esplanade in the Downtown Disney District. The lockers are a great option if you purchase souvenirs during your visit, or just want to store your bags throughout the day. You cannot leave anything in the lockers overnight, so you will need to collect your items as you exit the parks.
I hope this information helps answer your question, Phil! I'm wishing you and your family a wonderful trip to the "Happiest Place on Earth," don't hesitate to
come back and ask me any other questions you may have!
Have a magical day,
Melissa