I would suggest that you post this message on the Barefoot Herbalist forum. (although he may answer weird things at times, he offers interesting insights).
For what I've understood so far here on the Cure Zone, tooth decay is not caused by not brushing teeth but by eating the wrong types of food. I.E. eating Sugar
causes tooth decay not b/c it corrodes enamel in the mouth, but b/c it disrupts the body's chemistry and mineral balance, no matter how much you brush. Starchy foods such as cereal are well known to cause acidity in the body which in turn results in calcium deficiencies and therefore tooth decay.
correcting baby's diet introducing calcium rich foods such as raw broccoli and cauliflower, almonds and maybe raw dairy may help, BUT, 1st thing I would do if I were you, would be go to a dentist and get a diagnosis. It may not be what you think. Go make sure.