I was able to find the following information:

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Can fluoride be dangerous?

Fluoride is safe in the amounts provided in water supplies but can be toxic in large amounts. Toxic levels are proportionate to weight. A lethal dose of fluoride for a 3-year-old child is 500 mg and would be less for a younger child or infant. Keep all products containing fluoride, such as toothpastes and mouthwashes, away from children. 3 If you think your child may have swallowed too much fluoride, call your local poison control center or the National Poison Control Hotline immediately at 1-800-222-1222.

Too much fluoride swallowed during the years of tooth development may cause spots or streaks of color on the outside of the teeth (fluorosis). Most of these are white, although some may appear brown.

Fluorosis develops during the first 8 years of childhood while the outer enamel layer of the teeth is still growing.

Fluorosis is not harmful. In rare, severe cases of stains caused by too much fluoride, a dentist may bleach the teeth to remove stains, or may bond resin fillings onto the tooth to cover stains.

Studies show that normal amounts of fluoride added to public water supplies and in oral health products certified by the American Dental Association are safe for children and adults.

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It seems that the streaks in her cause were probably caused by flouride treament at her dentist. She has never been on antibotics nor do we have flouride in our drinking water.