Miss Manners suggests that while many are accustomed to providing wedding registries, the couple's desire for personal gifts doesn't negate the fact that relatives want to celebrate in some way.
Because you have important life lessons you want to impart, consider writing them in a journal or recording them. Since you seem to be making all the effort to see your great-grandsons, perhaps it's time to concentrate less on your children's children and put more effort into socializing with contemporaries.