It turned out to be "Madagascar 2" -- a slightly annoying cartoon about a dancing lion, though the kids all around, including Gwenchik, loved it. I remember walking along the water, sprawling on the blanket with a few snacks in anticipation of the movie, then, after sundown, the large screen and the loud speakers transforming the Esplanade into a bit of magic.
Then, after the movie, walking across the bridge to the MIT side to find our car, watching our beautiful city lights reflected in the river and counting "how many smoots are in that bridge" -- priceless.