The parents of the first case knew he was sick, which is why he went to his pediatrician. They likely didn't know it was measles, but maybe they should have since they made the decision to not vax and should have known he would be more susceptible to infections like measles.
The reasons we likely don't see any lawsuits is you likely have to prove that a person knowingly spread the illness. And the vaccine exemptions in many states probably provide some cover.
I'm not certain how a parent would be expected to diagnose measles as the cause of illness as opposed to any other viral exanthem. I wonder if there might be liability on the part of the pediatrician for having the child come into the office/waiting area with other children. If the child had not been diagnosed with measles then it's not likely the parent could have knowingly spread it.