Welcome to planDisney, Patrick!!!
It sounds like you're getting your 
Donald Ducks in a row before your visit to The Happiest Place On Earth this summer.
Well Patrick, I've got good news for ya: According to the 
list of prohibited items, as long as your backpack doesn't have wheels and is not larger than 
24" x 15" x 18", backpacks are highly welcomed at the resort. In fact, I regularly stuff mine with goodies (okay, mostly diapers and wipes) like snacks, portable batteries, handheld fans, 
bubble wand refills, and a 
refillable water bottle for the day.
I hope this answers your question and helps you prepare for a pleasant day at the resort this August, Patrick. If you have more questions or need more 
tips or 
suggestions, just 
shoot me a message here and I'll zoom back with an answer faster than Lightning McQueen can say "
Ka- Chow"!
Your pal,
Danielle