At pier 2 of the Brooklyn Bridge Park there are a few things to do for kids.
The great part is that it is covered, so you will be dry when it rains, and out of the sun on hot days.
They have nice bathrooms, more then they have at pier 1 and pier 6, so usually no lines.
There are exercise equipment that the kids love, its like toys to them, there are swings, many pick-nick tables, grass fields, and some more.
Too lazy to walk back to the train station? There is a city bus leaving from pier 1, it takes you straight to Boroug Hall train station.