The other night, I go to hang out with all three of my grandkids for a few hours. Their parents headed out to see the lights in St. Augustine, my wife had a meeting, and they wondered, “Can you watch them for a few hours?” Without a moment’s hesitation, I replied, “Yes!” I don’t get that kind of opportunity very often.
At ages one, two, and three, they really don’t play together. For the first half-an-hour or so, each wanted to play with what another had, but eventually they settled down with their own. After a bit, time for some music. I got out my guitar to sing songs, but no one really wanted to sing with me. Instead, they wanted to run laps around the living room, chasing or running from me as I played. Then it’s time for food.
Tortellini (with “dip”), strawberries and green beans were a hit, everyone wanting seconds. Once supper is over, a few episodes of Paw Patrol kept them quiet for a moment as I cleaned up supper.
My wife was the first adult human to return home, amused and perhaps a bit dismayed at the number of toys strewn around the house. Cars, pretend food, zoo animals, farm animals, action figures, balls, flashlights, a doll house, trains, pretend ice cream cones, kid-friendly power tools, books, lab coats and medical equipment, dinosaurs and a pretend Keurig are all needed to keep everyone busy for a while. Yep, we used them all! (Side note: it doesn’t take three to get and leave out all the toys. One can do the job just as well!)
As a grandparent, I relish these moments not only because they are fun and fill me with joy, but because I know they are rare. As a parent, you feel like you’ve got a lot of time to spend with your kids as eighteen-plus years stretches out in front of you. But as a grandparent, you know those years will fly, so you savor those moments when you have them.
Waite for it... 
Having just spent two days with my grandchildren I totally concur with your final observation.