Sounds like it's coming in with weak enamel? Some of my oldest's baby teeth were like that, little incisors so they fell apart fast. You can support tooth health with bone broth, hours long simmered bones like from roasted chicken or beef, use a splash of vinegar in the water to help break them down. Use in soups, sauces, or as a warm drink. Give nuts, greens, and they say to use vitamin added CLO and butter oil. You can look for a Hydroxyapatite calcium supplement that's the most effective kind for bone and tooth rebuilding. The controversial flouride ion treatments basically are to have flouride take the place of missing calcium, as it makes a harder than natural shell for the tooth. A good layer of calcium does a fine job though. I would watch carefully for cavities and fix them early if they start, and look at pros and cons of having the dentist apply a non BPA dental sealant when they are fully grown in.