Since the parents are divorced, I'd guess that your ex DIL looks after her parent's invitations to events, and your son is expected to look after his family's invitations. You might be providing child care for both parents benefit, but at the end of the day they are divorced so don't expect the ex DIL to include you in anything. Talk to your son if you want to go to things.
You are keeping close ties with your grandchildren by doing this child care - keep it up and enjoy the children. Don't let yourself get upset about these perceived problems about "graduations" etc - we all walk on eggshells , especially when there is a divorce situation. Paste a smile on your face and say " how lovely ! Do you have some pictures ?" And leave it at that.