First and foremost, congratulations on becoming grandparents.
I get the feeling this is your first grandchild.
If so, perhaps you have forgotten, as we all have over the years, how overwhelming becoming a mother is, so before you say anything to your son or DIL that could cause hard feelings, try and remember EXACTLY what scared you silly when your son X numbers of years ago was new born.
Looking back, you doubtless can see how unrational your fears were then, but at the time? Come on love: were you scared silly, someone would come into the maternity ward bring meningitis or TB or polio with them and give it to your cherished baby? Or were you convinced a horde of unhappy women who had just lost a baby, or who would never be able to have one were lurking around waiting for you to leave your son for a minute, so they could steal him?
This is where your DIL is now - scared silly that Covid is going to come along and kill HER BABY.
Cut her some slack, please. It will repay itself, I promise you.
Quite honestly, I feel that the risk of catching Covid on a plane, train or bus, where you are sharing the air you breathe with total strangers, going to the same tiny toilet cubicles, standing in queues etc. is far greater than if you drive there - even if a nineteen hour journey means staying overnight in a motel.
But it honestly does not matter if the risk is greater - your DIL is scared and convinced that it is. Humour her! She is not trying to hurt you or treat you unfairly. She is just a young mother coping with the emotions that flooded her when she held her precious baby and with the stark fear that invaded her when she really realised for the first time that from now on her first priority is to protect that little child, cost what it will.
Drive to see them, breaking the journey on the way, and if you and your husband can afford to do so, stay a couple of days and take covid tests in the town where you son and DIL live before going to see them.
Explain beforehand that this is what you intend to do, to be sure you mimimize the risk as far as humanely possible, and enjoy the visit and you grandchild, when you get there.