I think if someone is polite and calm during a request, she will get much further than the one who screams about how she's entitled to a credit. Not saying that's you ... just saying, from the many threads I"ve read on the topic of credits, that seems to be how it's done.
I do agree with a previous poster, however ... you should get either your nephew or his parents to pay the charges. It's his fault, not Verizon's. I do not understand why your husband would let him play with his phone.
Good luck. I hope you get results.