I'm assuming that the child already has permanent adult teeth, so that we are not talking about teeth that will be replaced in the natural course of time, regardless of their current poor condition.
Pits in a person's teeth are sometimes benign. An unfilled cavity can remain stable for a decade before it finally becomes so deep that it must be filled.
This is not to contradict your new dentist, because I'm not a dentist and I can't diagnose. Rather, I'm trying to show that there is some room for dispute as to what makes for a cavity that must be filled, and one that is simply too shallow to be of any concern -- assuming that the patient has good dental habits (brushes regularly, flosses, etc.).
The counterargument is that if these cavities are all of the sort that should have been addressed previously, and may indicate other issues, such as a need to change diet or brushing habits, use fluoride supplements or otherwise, then the existence of 20 cavities suggests that the former dentist may have been ignoring problems for a long time, and thus helping to cause the additional cavities. And, that is malpractice, because it falls below the care that a similarly situated professional would use in similar circumstances.
In order to sue, you would need a second opinion from another dentist who can testify to the former dentist's apparently substandard care, and to the fact that there is little probability that all of these cavities suddenly appeared between the patient's visit with the former dentist and the latest visit.
Unfortunately, that will cost money, because expert witnesses don't testify for free -- and even if you just subpoenaed the new dentist to testify, the court would order you to pay for the reasonable value of his/her services in testifying as to the child's condition.
Also, unless the child is mature enough to file a lawsuit on his/her own and testify in court, then you would have to hire a lawyer to represent the child, even in magistrate's court, where a lawyer is frequently unnecessary.
Those are the issues you confront. However, an appropriately drafted letter from an attorney may be able to obtain a quick settlement from the former dentist's professional liability insurer -- because the cost of defending the dentist in court probably would overwhelm the damages to be recovered. Under the circumstances, I think that you will get a better result here by hiring a lawyer, than trying to sue the dentist in magistrate's court without one.
For a civil lawyer referral, see this link.
Hope this helps.
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