My cut off point was a 'straw that broke the camels back' situation, however, I had unconsciously been very low contact with my parents for about 2 years before going no contact. After my golden child brother had a child my DD's were cast to the side and my mother especially started to treat my DD's as she and my father did me when I was younger. I had been doing a lot of reading regarding toxic parents and I found after all the reading and research etc that no contact was quite easy.
My parents, more so my mother, resisted and pushed back tremendously. However, I refused to have any contact whatsoever ever with her. I threw away letters, cards, gifts etc all unread and unopened (never sent them back to my parents), I blocked her on my mobile and eventually changed my number, I rebuffed all of the flying monkeys and refused point blank any, however small, contact with her. My father, being the gutless person he is, dropped me also. I knew that the possibility of losing him and other family was high, it still hurt though.
I am 5 years NC, I have had over 10 years of therapy, been in crisis twice and had 2 breakdowns, I suffer with severe PTSD, anxiety, OCD and depression (all professionally diagnosed by psych). I am only just starting to come out of the other end with the support of DH, DD's and my friends and a LOT of hard work on my own part.
My advice to you, read and research as much as you can about toxic parents/relationships, allow yourself to feel angry, upset, bewildered and any other feelings that show up. Understand that it is them that are wrong and toxic, not you. As PP's have said shit parents really do make shit grandparents so don't fall into the trap of thinking they will treat your DS well or better than they did you because they won't. You may have to challenge every thing you know, every feeling they incite in you because you will have been conditioned from an early age to put up with this abuse, and it is abuse. Don't let your DS become another victim of their abuse.