Honestly, with the information you have given, it would be irresponsible to direct you to a breed.
How active are you? Do your grandchildren live with you? Do they visit often? Do they have a dog at home? If so, what kind? What activities would you like to do with your dog? Will it simply be a housepet or do you walk, bicycle, fish, hike, swim, etc? Are you willing to take on the responsiblity of the 15 to 20 year life span of a toy breed?
Where do you plan on getting said puppy? Do you want to do the research on the breed and breeders before getting a puppy? Or do you just want to run to the local pet shop and pick one up? (That would be a HUGE mistake, by the way!) Do you want a puppy or an older dog? Have you considered rescue?
My best advice would be to go to your local shelter and see what is available. I am sure that you can find the perfect dog for you and your grandchildren without having to worry about a specific breed.