On the north side of the playground is a smaller structure for the little ones, a sprinkler and a sandbox (shaped like a boat, of course). The smaller structure is also ship-shaped (this one a bit more like a tugboat) and includes a slide, two types of ladders and music "switches".
One thing that Main Street playground is lacking is accessible restrooms. At the moment the closest thing is only a port-a-potty at the end of Main Street. To get somewhere where you might actually consider letting your child sit down, you'll have to either venture up Main St. to the Starbucks or otherwise walk south on Water St., past the River Cafe and Ice Cream Factory to a trailer with public restrooms that are surprisingly clean and spacious.
Potties aside, Main Street playground is a great way to compliment a trip to beautiful* Brooklyn Bridge Park and DUMBO. This one gets a solid A.
*Just to note - the north side of the park is currently under construction so THAT part of the park is not quite so beautiful at the moment, however, the part closest to be playground is still open and as lovely as ever.