hmm
Writing a letter, i guess you hoped so much for them to see the error of their ways, or recognise that they have hurt you?
By writing it down you have put it out there in the open and they havent responded how you wanted them to. It was a risk you had to take. I think you now have to try and accept that they and their behaviour isn't going to change over night or even at all.
For now you will have to make an excuse to your dd, because I doubt you will hear from them before xmas, as you havent done yet.
I do understand, i did letter to my parents about 10 yrs ago. Yet i was not a parent myself so didn't have to think about any effect on anyone cept myself. My parents did respond, havent changed much, but i have managed to move on and over these ten yrs accept them more. Mostly, accept myself and say to myself i am worthy.
For the moment i should concentrate on xmas, on your xmas, on family you do see, and friends you can lean on and share with.
Your parents, i can't speak for, only you can know how they would have reacted, what sort of people they are.
op did you truly expect them to react how you wanted them too, with a view to reconcilliation? Did you think that maybe they would be angry for a while. Did you think they might just not understand? I am not sticking up for them. Maybe they will send your dc's something this week? How can they have a relationship with their dgc's and not you?