Ask them questions.
Find a subject you are familiar with eg NGO/UN fiasco, 144k doctrine, blood doctrine whatever.... and keep asking them pointed questions. Research the topic first here to get some good points, then ask your parents questions that they can't answer.
I remember reading somewhere that you're not baptised, is that right? If so, the onus is on them to convince you that the JWs have the right religion.
Hopefully, in trying to answer your questions, you can bring up ideas that might make them think.