If you didn't give your kids fluoride when they were little, (if your water didn't already have fluoride in it) then it is too late for the fluoride supplements. When we go to the dentist, they do give fluoride treatments, but the most important time to have fluoride is as a small child, up to 12 or 13 years old.
You need your pediatric dentist to explain these lines. You can't find that out here.
When we were kids, long ago (LOL!), kids had mouthfuls of cavities. There wasn't any fluoride in the municipal water. When health agencies started to understand the benefits of fluoride in the water, cavities became less a standard of oral issues, and nowadays, not nearly as many kids have a mouthful of cavities. Indeed, only one of my children has a filling in his mouth (and ONE filling at that.) However, my other child had 4 baby molars without enough enamel (indeed, not even enough for sealants to stick to) and we filled these until they finally came out. Thankfully we don't have that problem with his permanent teeth. And this kid has no fillings now.
That being said, other than fluoride from little on up, the other thing that "counts" is genetics. Sometimes bad teeth run in the family.
You sound like you are really trying hard to help her with good oral hygiene. Take her to the dentist and find out about these lines. If she needs fillings, you need to get them regardless of whether you want to or not. Leaving teeth with decay in them will just make it so you have to have more difficult work down the line.
Good luck,
Dawn