The point of the story is that the kids are learning that they will go into the forest where there are monsters, and as clever as they might be, at least some of them will die there. Fortunately, that day was just a game. The lost ribbon is the play equivalent of a fatal wound; a torn jugular doesn't need special symbolic weight or meaning, it's just a torn jugular. It's a tool to the story, not the focus of it. It didn't have to be there any longer than it was.
I mean, if it had ended with the kids stabbing a beast all Shadow of the Colossus-style, and then the next panel was one of them with his chest badly messed up from a claw rake, no one would complain that the wound was devoid of symbolic meaning and that it wasn't built-up enough. Yet the lost ribbon plays the exact same role, only transposed in the context of a game where nobody actually gets hurt.