It's not as bad as one might think.
Yes, there's all the standard stuff.... burgers, fries, hot dogs, and so on. But almost every restaurant, even the quick service ones, have healthy options. I did have a cheeseburger and fries one night(as did the kids). But the rest of the time we had healthier things.... like a grilled chicken wrap and apples in place of fries. I only had soda once, the kids didn't at all.
I was also pleasantly surprised to find a lot of fruit stands throughout the parks. They also had whole wheat pretzels.
One way we made our own lives easier was to bring food with us. Two of the days(out of four) we packed our lunch and snacks ahead of time. We rented a locker at the park and stashed our cooler there and some of the 'extras.' We kept bottled water and healthy snacks(apples, healthy crackers, etc.) in the bottom of the stroller, so we could have them whenever we felt like. This also saved us a LOT of money! And without all that bottled water, I know we would have had a lot more soda!
There is also no question that food is expensive, but the quick service stuff(as opposed to the full service sit down dining, of which we did NOT partake) is not as earth shatteringly expensive as one might think. It's definitely still best to buy as little as possible while there, but if you do need to(or choose to) eat, it won't completely break the bank. I think the reason it all seems so expensive is if you're entirely reliant on the park food for three meals a day plus snacks for the duration of your stay. Yes! That WILL add up, but we stayed off the parks, and had a whole kitchen in the condo we rented. We ate breakfast there every day before leaving, and that helped a lot.
So now that I've rambled.... It's expensive(sort of), but can be healthy pretty easily!