From what I've read, it wasn't that he had a knife on him (came equipped).
It was worse. He pulled the knife out of the sand. Which means he planted it there before he invited the other boys to the park.
BTW, here is a Google Maps view of the entire scene, including the victims home and the "playground", which is not actually in view because it was evidently added since the Google image was made. The scene is nevertheless identifiable by the shed-like building at the entrance to Pinebrook Village. In the Google view there is a dumpster in the alley there, and the climbing set is near that along a fence between the PV property and the back of the long house to the left. This house or one next to it was the victim's home, based on news reports with the cop cars and crime scene tape.
Also BTW, note the photo of the perp as a young boy which has proliferated. I was thinking he is a lot younger than 12 in that picture. Then when I watched a Fox 17 video on "neighbors' reaction", the "young" photo was shown several times, then right at the end ( 2:33 out of 2:36 ) what looks like a police photo pops up and is visible for just a second or less before the video ends. Very strange, and quite a contrast, if that's really him, and I believe it is.