My wife and I, both in our late 60's have a married daughter with 2 children living about 100 miles away.
Our parents taught us that you just don't drop in friends or relatives but wait for an invitation.
Our daughter says we are always welcome & do not need an invitation. Consequently this is causing tension in the family. We never get an invitation for any occasion.
How should we address this?
Ah, the variations and vagaries in generational etiquette.
The "drop in any time" works fine when you live a few minutes away and are buzzing by daughter dear's house frequently. When you see them out on the porch on your way home from the grocery store, say, or if you want to drop off some surplus lasagna you certainly can pop in for a bit. But for distances like yours that model is completely untenable.
Assuming you and the Mrs. do want to visit daughter, son in law and grandchildren with any regularity, you are going to have to adjust the tenets of your very good upbringing. When the mood strikes you to visit the kin, shoot them a call/email/smoke signal indicating your desire for a get together. Offer a few dates that work for you, and let them know that you are flexible and willing to work around their calendar. Since they have set up the protocol that they do not issue invitations to their home you will have to be a bit more assertive than strict, traditional etiquette would suggest.
Another tactic to employ: ask them when the kiddies have events that you could attend--ball games, school plays, music recitals, dance performances, tiddly winks championships, etc.. Let them know that you would like to come celebrate these occasions with the family and support the offspring in their pursuits. In this vein, I send my parents the softball schedule before opening day, same goes for dates of any performances or recitals--emphasizing that there is no pressure to come, but offering it up for their consumption if they wish to attend. When time permits, they do come and the grandkids are thrilled. Few parents will decline the offer by grandparents to swell the applause when their budding Mozart pounds away on "twinkle, twinkle, little star."
Finally, when holidays or other momentous events are on the horizon, throw your hat in the ring. Say, "Daughter, Thanksgiving is drawing near. We'd love to share a turkey with you and yours; can we get together?" If hosting is difficult for them due to the size of their home, the age of their kids or their level of neuroses, consider inviting them to your home or visiting them but staying elsewhere (a local hotel or B&B). This enables you to enjoy quality time without overdoing it.
Remember, this is about maintaining a relationship with your daughter and son-in-law, and cultivating one with your grandchildren. Standing on ceremony here may leave you standing alone when you'd much rather be standing room only at your grandson's chess tourney or standing at the buffet at your son's holiday party.
Good luck and thanks for writing!
And speaking of grandparents, I came across this hilarious clip--not exactly on point with this post but it made me laugh out loud. So, yes, Grace does have a low brow sense of humor on occasion.