1. There are other reasons besides tooth decay that diet sodas are bad for you. I would never give my kids diet soda. Never ever.
2. Soda is very acidic. If you were to soak your teeth in soda, it would eat away the enamel. But drinking soda is hardly equivalent to soaking your teeth in it. In moderation I think it is fine.
3. Apparently the presence of sugar in the mouth provides a nice food source for bacteria that cause plaque to form and tooth decay to occur. Obviously if you never brush your teeth to remove the sugar and plaque, you will have problems. Nowadays with toothbrushes, floss, fluoridated toothpaste, and modern health care, I don’t think this is that big a deal.
4. Some people have thinner enamel than others. Some people are more prone to getting cavities than others. If this is true of your kids, then extra care in limiting their exposure to sugar may be wise.
5. It isn’t really very healthy to be ingesting loads of sugar regularly anyway. I believe that the effects of high levels of sugar in your body are cumulative, kind of like UV exposure. As sun exposure can lead to skin cancer, a high-sugar diet I think can lead to serious health problems like diabetes. I’m not saying that kids should never drink soda. Just something to think about.