You have no say in the type of medical/dental insurance he carries
You have no say in which company he chooses to insure the child
There is nothing barring him from switching carriers, nor is there anything that says the policy has to be the same or better.
In fact, if he can prove he can't afford it, he doesn't have to provide it at all. The code says he is to provide it IF it is available at low or no cost to him. He doesn't have to break the bank providing insurance you like.
You have no cause of action towards the dentist, because it is YOUR responsibility to verify benefits before treatment is rendered. The office simply bills insurance as a courtesy, they don't HAVE to. Oh, and if this isn't your policy, they don't have to speak to you or do business with you at all.

