Locked room for breakables when grandkids stay?

Either move things up above the child's level or pack the precious breakable away when they stay.
Unless you live in a mansion of course in which case lock the room.

I must be lucky because none of our lot have broken anything much except the odd glass tumbler by mistake. My kids didn’t ,either. Don’t need to tell them.

Looking to the future, you might need to rethink the location of your collection or you'll be a nervous wreck whenever the family visits!

I suppose it depends how big your house is, and if closing off one room is going to make the rest of the house crowded.
We have a large collection of glass and, touch wood, we haven't had any breakages as the grandchildren were taught not to touch. We don't allow indoor ball games either.

Not that.

If the 3 year old breaks something of sentimental value I would need it and it would be part of our family history.

Like the time 7 year old (play) wrestled his pa to the floor and hurt his wrist. It's families.