I'm a bit on the fence about this. Ultimately, OP I think you have to decide what would sit best with your conscience and don't give anything which is done whole begrudged. Going against your conscience will eat away at you and doing something you utterly begrudge will just leave you bitter and resentful. Both will be more of a blight on your life than anyone else's. Only you can decide where that line is for you.
Although I do find some of the bitterness and entitlement displayed on the thread quite odd.
I had a good childhood, some ups and downs, but on the whole fine- it isn't like my DC's life as expectations and opportunities were quite different then and I wouldn't try to do a direct comparison. OP's childhood sounds quite similar and it seems she doesn't think her childhood was a bad one. Her gripe is that her parents haven't provided childcare or financial support as she might have hoped and feels they were selfish in prioritising their own needs and wants over her/their DGC. I'm not sure that deserves a "fuck them" as some posters have put it. Or that you just cut them off or ignore them.
I think some of the examples given by OP were selfish- e.g. not helping with childcare to allow a one off opportunity for their grandson because they wanted to go the cinema. So I can see that they haven't been the most supportive of parents and that OP could have wished for more from them. So I can understand some of her feelings. I don't know that not providing her with the money to fund a car comes even though they could have afforded to do so comes into the same category.
I think it would be reasonable to do some things, if OP is free to do so, but I don't think it unreasonable to say no to full-time or frequent care. An occasional lift to a hospital appointment, if free and not too inconvenient, wouldn't seem like too much to me. But if OP doesn't want to, then she shouldn't.
However, I do think parents ARE entitled to enjoy their retirement and spend time doing as they want without being committed to childcare (either regular or whenever it's decided they are needed) or having their spending examined by their adult DC to decide if they have been to extravagant and should have given their DC the money instead. I find it odd to consider yourself entitled to your parents time or money (or if not entitled, hard done-by if you don't receive money which you feel they could spare and would help you out). Surely you should stand on your own two feet by the time you are having children? Helping out in a crisis is one thing- supporting adult DC's day to day spending or more routine things such as funding a car is not something I would expect to be doing for an adult child (perhaps a first car, but not really beyond that) and not something I would expect from my parents.
I had a very close relationship with my DGM. And she did not do regular childcare for my mum, although she did live several hours drive away. We visited during summer and october holidays and we would see them at christmas too. When we were older, they took us for a week in the summer holidays. But that was it. You don't need to do regular childcare to build a close and loving relationship with your grandparents. I also think you have to bear in mind how many DGC they have. You may only be asking for a day or two- but what if they have 2 or 3 sets of DGC?
My DM has done quite a lot of childcare for me (I'm very grateful and she volunteered) and has a great relationship with DC. My DF less so and hasn't done a huge amount of childcare himself (has been around when DM has been there). PIL have done far less- partly because they have 13 grandchildren and partly because they have really busy lives. I don't think that you can expect heavily involved childcare for 13 DGC, personally. I think they aren't as close to DC as my parents, but I do think they love all their grandchildren and don't hold it against them like it is some sort of competition or "minimum standard" to be marked against.
I don't think I have ever sat down and worked out how much care I would do for each set of parents, based upon how much childcare they have provided or how much money they have given us. I won't be moving either set of parents into my home or doing full time care because I work full time (and currently have primary school aged DC) and don't think I could cope with full-time caring- though my parents do live in a "granny cottage" on our property. And I don't think either set of parents would expect that. I would help in a crisis or short-term if needed- not because they've helped me but because I love my parents and I wouldn't want them to suffer. TBH, I have given an elderly neighbour who was struggling to get to hospital appointments a lift and I didn't know her that well. I didn't expect something in return. So I'd probably do it for a parent I wasn't overly fond of. But I would draw a line about what I feel able and willing to do- just as my parents/PIL have done and just as OPs parents have done.
On the flip side, I don't feel entitled to my parents or PIL's money or begrudge them spending on themselves and doing things they enjoy. So if they choose to spend it on a nice car or holidays or a new kitchen or whatever- that's fine because it is THEIR MONEY. Not mine to covet because I am not as well off and be resentful that they are able to enjoy life while my finances are tight. When I think back to where my parents were at a similar age to me, they were in a similar position to me really. They certainly have more spare cash then they did then. They managed, and cut their cloth to match their means. I do think some people don't seem to be able to do that very well and want the "bank of mum & dad" to pay for choices they make.
Ultimately, surely part of growing up and becoming an adult is taking responsibility for your choices? That includes budgeting to live within your means and not expecting other people to support your life choices by giving up a proportion of their retirement.