N.S.
That's so sad that the kids are being so mean! My heart breaks for your little girl.
First of all, you should not worry if your 6-year old has a belly. My 8-soon-to-be-9 year-old has a belly and has had one since she was a baby. It just never went away. We've been assured that it's perfectly normal and will go away as she grows taller.
I agree that you should not focus on food with gaining weight, but focus on health. At age 6 she can learn about the food groups and the food pyramid (get a book out of the library) and you can discuss how EVERYONE needs to eat healthy. Don't bring up weight at all. Then she can help everyone eat healthy. With health comes exercise, perhaps you can enroll her in some kind of healthy activity like soccer or swimming.
As for something being wrong, definitely seek another opinion from a kinder doctor! Perhaps there is something wrong with her thyroid. Or maybe she just burns a lot of calories! A second opinion would put your mind at ease.
Kids will make fun of kids, and they'll pick out whatever sticks out. If you're skinny they'll call you "stick" and "beanpole." If you're ethnic they make ethnic remarks. If you have red hair they'll make fun of that. Just let your daughter know that it's not true if she doesn't believe it. It's a good lesson to learn to have thick skin and to recognize that NO ONE has the right to determine who YOU are. Only you get to do that!
If she is a little chubby, don't outright tell her she isn't because she'll know it for a lie. Just like you wouldn't tell a redhead that her hair isn't red! Just focus on how kids are being mean, they have no right to treat her that way, and coach her on what to say in return. Also let her know that she is who she is, and she can't change that. But she can change who she is on the inside :)
Good luck!