If it is fullsize back pack, get a locker inside the park. OR if you have little ones, take the valueables out, and leave the back pack in the stroller. Which mean syou would need a fanny pack or smaller size back pack as well.
A smaller backpack can be brought on rides. Some rides have a pocket to store, others, you can hold it. For example, on BTMR or ToT, I put my leg through the backpack straps and sit on them and let the back pack go between my feet and I am fine to put my hands up!