This working is a function of density. First, imagining a park at the corner is a huge assumption.

Second, if the park isn't within shouting distance of the necessary number of kids, half of the ones within shouting distance are at parent-scheduled activities, and half of the remainder aren't allowed to brave street crossings because of the six-foot-tall hoods and inattentive drivers of passing ChildMurker1050s, no amount of park yelling will be sufficient.