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.
If you want to offer an explanation, let your family members know after the trip that in an attempt to preserve the surprise for your children, you waited until you were already on the way before telling them... I'm sure some family will say that they would have gone if you'd told them, or that they wish they could have been there at the same time, to which you can express your own "regret" and say, "Yeah, maybe next time."
