I think it is always worth another try, using any opportunity of contact you have, so if you have their address, with a heartfelt letter. My approach was this….no accusations or bitterness, focus on them not you, don’t ask for anything, don’t make it about you and what you want, apologise unreservedly for anything you realise may have upset them. Make it crystal clear that you would act differently in the future, given the chance, and that whilst your grandchildren are hugely important to you, there is nothing more important than repairing your relationship with your child. Most estranged parents will have been given clues as to why the estrangement happened. If you weren’t listening then, as I wasn’t, think back over those clues and listen to them now. Put yourself in your child’s shoes, literally turn the situation round and look at things from their viewpoint, and see how things felt to them. If you can see their point, be honest, admit it & apologise.