One of our favorite playgrounds is in Brooklyn’s Dumbo neighborhood. It comes with a big downside, though: there are three gates, and it’s impossible to see all of them from a single vantage point. If you happen to go there with a free-spirited five-year-old and a two-year-old, you can find yourself in a tricky situation: The older one takes off suddenly and heads toward the river, and you have to leave the younger one by himself while you fetch the escapee. However, you can take advantage of a boat-shaped sandbox in the north part of the playground, whose sand level is so low that the kids are surrounded by an insurmountable three-foot wall. (Disclaimer: I always made sure there was another grownup at “the Grotto,” as we liked to call it.)