I would say that the best line is this one:
"They had quite forgotten him."
A few paragraphs earlier though, the reader is given some hints that the other children have forgotten about Ravi and moved on to another game. Two words let the reader know a lot of time has passed. "Evening" and twilight." To go from afternoon, to evening, to twilight on a single round of hide and seek is a long time. Most readers would know that from experience and already question whether or not any of the kids are still looking for Ravi. At the end of that same paragraph comes these lines:
"Could he hear the children’s voices? It seemed to him that he could. It seemed to him that he could hear them chanting, singing, laughing. But what about the game? What had happened? Could it be over? How could it when he was still not found?"
The author is dropping hints that the game might indeed be over and that the kids have forgotten about Ravi. "Could it be over?" is a strong hint.
Just before the line that says Ravi had been quite forgotten is this line:
"It took them a minute to grasp what he was saying, even who he was."
Ravi was so removed from the other children's minds that it took them effort to remember who he even was. The end of that paragraph provides the most hurtful detail that nobody remembered him and that he was playing the game.
"All this time no one had remembered Ravi. Having disappeared from the scene, he had disappeared from their minds. Clean."