There are a couple of things you should be looking for. You may think she isn't eating sugar, but she may be getting other forms of sugar that you don't realize can be causing cavities. You should watch the overall refined carbohydrates, and fruits while they are healthy are also a source of cavity causing sugar, especially dried fruit, which is the worst because of it is also sticky.
The other concern, besides sugar, is acid. Many people, including children, have too much acid and acid will eat away the enamel of the teeth. You can buy pH paper or strips in the pharmacy and see if your child's saliva is acidic (below 7) or basic (above 7). Xylitol is an excellent sugar substitute that will neutralize the acid in the mouth and reduce the growth of cavity-causing bacteria. I personally recommend Zellies (zellies.com) gum and mints because they are all natural and non-GMO. My grandchildren have at least one mint (sucking candy) after every meal and snack.
Robert M. Hersh, DMD
Happy to help,
You should begin by rinsing several times a day with warm salt water, which will help reduce the inflammation. Next, you need to remove the plaque from your teeth. In my office, we strongly recommend an electric toothbrush along with a water irrigating device (on a gentle setting). At first, you will notice an increase in the bleeding, but as they begin to heal, that should diminish.
Next, I would recommend a visit to a dentist who can do a more thorough evaluation and help you treat your condition.