For as much as folks grumble, WDW actually makes it pretty easy to eat well.
At CS restaurants, you can always get carrots or fruit (apple slices or grapes) instead of fries with your meal and they have water, Diet Coke, and Light Lemonade for drinks. There are also usually salads and vegetarian options at most restaurants.
I have never found published nutrition info from Disney, except on pre-packaged foods, but you can usually estimate pretty well on the simpler foods.
A CS meal of 6 chicken nuggets, carrots or apple slices, and a no-calorie drink would probably run you about 6 or 7 points (more if you want a dipping sauce).
Another great option at CS is kid's meals. If you want something a little more "fun" like a burger, hot dog, or pizza, the kid's portions are just right. No one will bat an eye at you ordering a kid's meal at a CS. A kid's size hot dog or burger meal with fruit or veggies on the side would again be about 6 or 7 points.
At table service meals, the single best thing you can do is control your portions. Skip the rolls--you've had rolls before. They're not special at WDW.

Split meals, or literally just divide your plate in half and aim to only eat that. Ask them to modify your meal--many will give you fruit or veggies instead of fries or use less oil on the veggies. They're more than happy to help you out.
Try to avoid buffets if you can. It's just sooooo easy to overeat there. If you must, try to do them for breakfast. If you can just steer clear of the pastries, you can't do too much damage with some scrambled eggs, fruit, and a Mickey waffle.
Each park as at least one fresh fruit stand. A big piece of juicy watermelon is a great low-cal, refreshing snack that doesn't feel like "diet food".
Let yourself splurge on the things that really matter to you. If you can make good choices 80% of the time, the other 20% isn't going to hurt you.
I hope you have a great time!