Oooh, I don't think that this is an issue that I would bring up with parents. Presuming that they take the child to the doctor for well check-ups, etc I would say that if there are any medical concerns, they will come out.
I would continue doing what you are doing. Serving healthy foods in the right portions. I would NOT disallow her the treats etc that everyone else is eating. At two, she isn't going to understand why others get a cupcake and she doesn't. I would just stick to that portion control, as in "you can have ONE cupcake, not three" but I am sure you aren't giving all the kids 3 cupcakes, hahaha.
If mom/dad aren't in the most fit physical shape, then she probably isn't getting a lot of excercise at home, especially if she's an only child. Being at your house, playing with all the kids, can only benefit her!