I am always couting calories, so I'd be happy to give some suggestions:
There are boxed cereals and milk so you can count your calories. Fresh fruit is always available. Salads are too.
Avoid ordering fries at counter service and get the apples or carrots instead.
Minnie's ice fruit bar is only 100 calories and sold at the parks at the icecream carts.
In the Magic Kingdom, I get a baked potatoe (don't add the sour cream that they give ) and they have fruit, too, at the outside "market" across from the Riverboat. You can take it into the side door of the Columbia Harbor House restaurant for airconditioning, and if your family wants to eat in there.
At counter service restaurants, I get turkey or chicken sandwiches on wheat bread and use mustard instead of mayo.
My splurges are chocolate covered bananas

and jalopeno stuffed hot pretzels

and Dole Whips

, but I make them a "meal".
There are healthy choices in every park and you just have to use common sense and not get wrapped up in the thought that "I'm on vacation, so I'm going to eat whatever".
Check out the menus on Allearsnet.com.
Avoid funnel cakes and all the obvious high calorie items. Have someone in your party get something yummy and take one bite of it.