I think I have a really good idea. It seems you are leaning to help. That is of course fine. They are your parents. Their treatment of you isn't going to dictate what kind of human beings you are and that is virtuous in my mind.
Of course nobody would judge you adversely for not giving it to them (unless you are wealthy and have a bunch of disposable income).
I would offer very kindly if you decide to give it to them, to have them come over and eat dinner, and meet their grandson, and you will give it to them after the meal. This accomplishes so many things its hard to list.
If they say, well you know we can't do that, act surprised. I mean...they aren't suposed to be talking to you at all right? if they come, give them the money, then invite them over a couple weeks down the line for dinner again. Maybe your kindness will make a crack. If they decline and pull out all of the JW nonsense, ask them why it was ok 2 weeks ago?
I really think this is a good idea. it allows you to take the high road AND allows the opportunity for them to either see the light a little, or be exposed to THEMSELVES.
Make sense?