Don't worry yourself too much, this happened to my now 71/2 year old. I brushed & brushed her teeth but the dentist told me that sometimes it is inevitable due to genetics, possibly diet during pregnancy, & possibly my daughter's diet (on my side of the family, we just are not "big eaters".)
It was actually very strange, he told me that her teeth were "perfect" yet she had the same cavities/enamel wear on her two front teeth that you are describing(my daughter's went from white stains to brown). The dentist actually told me that it wasn't really necessary to have them filled unless the gums were swelling/bleeding or she had pain, I couldn't believe it! I got mad and sought out a second opinion, the second dentist said the same thing. But I thought it best to have them filled, so I did.
BUT WAIT, BEFORE YOU "PUT HER TO SLEEP", There are other options! My daughter had hers done by two different dentists. The first dentist used anesthesia & put her to sleep(I had to pay an extra $1000 for this), he did a poor job & the fillings soon wore off (I opted for porcelain that matched the teeth) so I ended up taking her to another dentist. The second dentist did not use anesthesia and told me that it was completely ridiculous for the first dentist to tell me that was the first and only option that I had, he said that especially for children that should NEVER be the first method tried when it comes to getting dental work done on them.
Anyways Those teeth have since fallen out & her permanent teeth seem totally stronger, I now brush her teeth with fluoride toothpaste & make her use a fluoride rinse, & I ensure that she takes a calcium supplements.