Ravi thinks that winning the hide-and-seek game is the most thrilling and wonderful thing that he could possibly imagine.
Ravi can't even begin to describe how awesome winning would be.
To defeat Raghu—that hirsute, hoarse-voiced football champion—and to be the winner in a circle of older, bigger, luckier children—that would be thrilling beyond imagination.
The thought of winning and the feelings that it would bring are "beyond imagination." That means no matter how great Ravi imagines winning might be, winning will be better than that. He believes that winning will earn him respect and adoration from the other children.
Ravi is desperate to experience that win over Raghu. He is willing to endure for many hours in order to secure his victory. Ravi's hunger for the win is why he stays in the shed for so long. He must be in there for hours. The children rush out of the house sometime during the afternoon heat, and they immediately begin playing hide-and-seek. Ravi hides in the shed, and he doesn't come out until twilight.
Unfortunately for Ravi, the feelings that he has when he wins are indeed beyond his imagination. Ravi never once imagined that his victory would earn him disdain, but that is what it brings to him.
"Stop it, stop it, Ravi. Don’t be a baby."