First of all don't listen to that last person. Truth is you can offer all the healthy stuff to your kids, and they will not eat it if they don't like it, if they think they wont like it. I naturally like healthy foods but my son got older (hes 2 now) and he will only eat chicken nuggets, peanut butter and jelly, grilled cheese, cereal, oatmeal, cereal bars.
The trick is to sneak the healthy stuff in the stuff she actually likes.
Make sure the chicken nuggets have enough protein (Tyson chicken)
Fries are ok some times, try sweet potato fries. Pizza is not that unhealthy, make sure there's veggies on it. Veggie chips in the health isle have no real nutrition, but there better then potato chips, and taste great. Definitely cut down on the sweets. There ok every now and then (special occasions, holidays) Chicken salad is very good, use light mayo. Try introducing her to real cheese, swiss, cheddar. ect.
I wouldn't give her anymore soda. That's the worst. Doesn't hydrate, only full of sugar and carbs. Gatorade, and water are fine.
I suggest you give her V8 Fusion, it has veggies and fruits in it but tastes like juice. Nutragrain cereal bars are good to. Use whole wheat bread for toast or sandwhiches. (wonder whole wheat bread)
Here'ss a little tip: Don't tell her whats in her food, or that its good for her. Just put it in her reach and let her try it. Always try new foods.
You kids are going to watch you and eat what you eat. Start eating healthy and only keep healthy foods in the house, and they will follow soon.
If your little cousin continues to eat like this, she will end up over weight unhappy and unhealthy. She has a strong chance of getting diabetes as well.