It may be that your kids are prone to cavities or maybe your dentist is being aggressive with treatment. Seeking a second opinion may be the only way to know for sure and I think it's time that you do so.
Ruling out other causes may help as well if the problem remains. In our case, DS2 started having multiple dental problems - like 8 cavities identified at one visit. Of course some of those spots were already noted and we were watching but it was awful facing 8 cavities at once. We too were doing everything right with brushing and flossing. Turns out that a medical issue was causing everything. Our amazing dentist started suspecting a larger problem and long story short, DS was eventually diagnosed with GERD. His teeth are looking much better - clear dental visit last week YAY! - after getting his meds and diet straight. Not saying this is your situation, but it never would have dawned on me that DS had GERD causing his problems. He didn't really complain and has always gained weight well.
Mom to Two Wild and Crazy Boys and One Sweet Baby Girl