Hi Alicia!
That's a great question! My son has been a picky eater most his entire life. He's 12 years old now, but we've been taking him to
Disney World since he was 18 months. He was a picky eater then and a picky eater now, so it's something we always have to consider on each Disney trip..
If you don't want to lug extra things around the theme parks, my first suggestion is to
rent a locker at any of the theme parks. The
Locker Rental areas are typically located close to the theme park entrance, and although prices are subject to change at any time without notice, a small locker at this writing is $10 per day. Unless you plan on storing a number of large-sized items, a small locker should be more than enough to accommodate some extra clothes and snacks for a 5 year old.
Now, although the lockers serve as a good option, they're not very effective if you're in a pinch, and need to change clothes (or give snacks) to your child and you're on the opposite end of the park, far away from the locker area! Small book bags and/or knap sacks are allowed. Since I was always the one in the family carrying the kids' items, I would get a book bag small enough to not feel like one of those over-sized hiking bags, yet have a little extra space to, say, store souvenirs.
Disney allows for bags, purses, and other items to be carried in the park. Many of their
rides you are able to get on without having to take off your back pack, and the one's that require taking off the back pack generally have a net holder to place your bag, or guest can place the bag by their feet when on the ride.
I do understand the reservation of leaving a bag on the ride, so you'll have to weigh what you think is best for you. If you're prone to losing or forgetting your bag, I'd go with the
locker rental. If you feel you can keep a right-sized back pack close to you, use a back pack. There's also back packs that are sold that the child can wear, that goes both over their shoulders as well as a strap that goes around their waist, so just another option to consider to keep from losing.
Have a magical time at the
theme parks and all of
Disney!
Terry