First of all, consider whose children they are. They are not yours. Therefore, it seems that if anyone is going to explain your divorce to someone else’s kids it ought to be those kids’ parents. Certainly, at the very least you must ask the parents if they or you would be the better choice. Even if you strongly suspect that the parents will do a poor job, it is still their job, not yours.
It is certainly an unpleasant message for grandkids to receive, but nowhere near as devastating as hearing their parents are splitting up. They will come to terms with it a lot sooner and as long as both grandparents maintain contact and a loving relationship, the grandkids will look forward to visits just as they had before…and the visits could be more frequent. There’s even the future possibility of each grandparent remarrying and adding more grandparents to the family.