Hi Melanie!
Welcome to the Disney Parks Moms Panel! I take it from your resEARching menus that you'll be visiting Walt Disney World Resort with a finicky
Mouseketeer? My three-year-old is also that child whom I constantly have to pre-plan our days around food-wise because he's in that stage of not-eating. Luckily, we've never had a problem finding at least one thing he'll eat at Walt Disney World Resort.
Yes, with a few exceptions, you are absolutely able to make substitutions for side items (and children's drinks!) when ordering either a kids' or regular combo meal. In fact, I've personally traded grapes for fries at Cosmic Ray's Starlight Cafe in Magic Kingdom Park.
The few exceptions I can think of are as follows: The dining location must already serve fries. Some locations, like Satu'li Canteen in Disney's Animal Kingdom, simply do not have fries, so double check a menu on your
My Disney Experience Mobile App before you stand in a long queue only to be disappointed at the register. Secondly,
Kids Power Packs, like this one at Catalina Eddie's in Disney's Hollywood Studios, might not allow substitutions because these meals are pre-boxed. And Thirdly, the Quick-Service Dining Location at your
Disney Resort Hotel may have multiple stations where fries are served in one line, and the entree you desire is served in another. Ask both the cashier and the order-taker at the entree station if fries from another line would be allowed as your side before you make your own combo. It's usually fine, but I'd hate for it to not be on your visit.
I hope this helped, Melanie, and We'll See You Real Soon!
~Kristen