https://disneyland.disney.go.com/faq/parks/permitted-items/
Things like:
Coolers larger than 6-pack sized are not permitted into the theme parks.
Alcoholic beverages
kind of imply that bringing in food is ok. I've gone twice and not had a problem bringing in PB&Js, snacks, and even a pizza (not in a pizza box - I'm not pushing my luck THAT far).