Reniaa brings up a good point. I do a lot Chrisitian activies volunteer work, etc.. My own Father cornered me on this once and said what do you do when they bring up the Trinity. I told him uhhh Dad, they aren't JW's only you guys constantly bring it up. We are just people that believe in God and accept Christ as our saviour, everything else is moot beyond that and subjective for the most part. Christ said not to be divisive but to be inclusive. He certainly didn't say all that is needed for salvation is to believe in me and the Watchtower did he? To speak directly to Reniaa, the Trinity is biblical. First you must understand that what you think Christians think the trinity is, is not so. Clear your mind of what the Watchtower tells you about the trinity and ask a few pastors or crusie the internet. Then compare that with what the Bible says not using the New World Translation which was translated by a spiritist x-communiated Catholic Priest. (See Johannes Greber).
That being said Password: I would relay that sentiment to your Dad. Tell him you just don't understand why God would find it so displeasing to him that people would choose to listen to songs about their relationship with him. You also need to testify about your relationship with God. It is subjective and is something not even a JW can argue their way out of. This will also plant those first seeds that maybe it is possible to relate to God outside of the org. Don't be anything but apologetic about not telling him. Don't say you are going to keep the grandkids away from him or show any other kind of dissapointment or negativity. Keep is sunny and positive.
When you hang up though... Give him the cold shoulder for a while make them reach out to you. They will call ask about the grandkids, say where have you been etc..etc.. Just say you've been busy that's all. They will come closer trust me. Nothing you are saying is earth shaking you are making them look like the jerks if they ignore you for "loving God". This is how I broke my folks. It's a process, good luck.