Hi dhomesteadma,
Childrens' nutrition is so critical. I'd suggest that, in addition to consulting your daughter's pediatrician, you check the Child Nutrition forum here on MedHelp and direct your questions to one of the experts here. Renee Radenburg, a licensed dietician, has a interest in Nutrition AND Childhood Nutriton. You can reach her & find links to the Child Nutrition forum @:
http://www.medhelp.org/doctor_profiles/show/267079 .
Peanut butter, low fat yogurt and tuna are good starts (though check with the experts to see if there is any need to limit the frequency of tuna in a youngster). It's great that she eats fruit, too. While whole fruits & veggies are always preferable since they usually have fewer calories and more fiber/nutrients, you might be able to "sneak" some veggies in using the newer fruit-veggie juice combos that taste like fruit juice. I also saw the wife of comedian Jerry Seinfeld on a talk show demonstrating a recipe where she "hides" pureed veggies (broccoli, spinach) in brownies (though you'll want to limit sweet treats); she has a cookbook that supposedly provides recipes "made-over" to include more veggies & nutrition (though I haven't read it myself so I can't vouch for it personally). Nuts offer fiber and protein IF allergies aren't an issue. If she eats pasta, you can select sauces that have a higher vegetable content. Low-fat cheeses can offer nutrition. Will she eat chicken at all---maybe as "finger foods"? Some kids do have food aversions and will gag with foods of certain colors, textures, etc.; sometimes they'll tolerate the same food prepared in a different way. If it is really a severe problem, sometimes they need the assistance of a doctor, dietician or psychologist to help them make behavioral changes.
In addition to diet, limiting her TV/computer time and encouraging her to participate in active pursuits is crucial. Is she interested in any sports at school, at the YMCA/Girls Club or in the neighborhood league? Can she just run about with the neighborhood kids, playing tag or kickball, softball or other activities? I know these opportunities may be limited. If she's significantly overweight, she may not feel comfortable being involved in organized sports and in today's world we don't always have the same close-knit neighborhoods where we can send our kids out to play and know they are safe. If this is the case, does she have siblings or other relatives who can engage her in physical activities---walking, hiking, flying a kite, riding a bike? Does your health and schedule permit you and or her Dad & siblings to take a 20-30 minute walk or bike ride with her after supper? Not only would it give everyone some exercise and set an example that exercise is something you value, it'd give you an opportunity to "touch base" with her about what's going on in her life. I know---we all have hectic schedules and some of this advice seems like "Pie in the Sky"---but if we substitute bike time for TV time, even if only twice a week at first and find out we can accomplish that, maybe then we can expand it to 3 or 4 or more times a week.
You are doing your daughter a tremendous favor by being concerned about her weight and nutrition. In just a few years, she'll be reaching those pre-teen & teen years where peers can be judgmental (and sometimes cruel). By helping her achieve a healthy weight, you can help her avoid some of that "teen trauma" (though there's so many sources of it!). By helping her be physically fit, you'll help her feel strong & develop confidence in herself and her abilities. Just be careful not to harp too much on weight or to be too critical. Some kids, even at her tender age, develop eating disorders so you want to approach things carefully and monitor her reactions and behaviors. Set a good example by eating well, exercising and avoiding putting yourself down if you are overweight at all. Let her see that weight, while important, isn't all that defines a person.
Good luck. You're a great Mom for seeking ways to give your daughter the best life she can have.