How did Anita Desai portray the child's disappointment in the story?

I believe that the child you are referring to in your question is Ravi. He spends most of the story hiding in a shed from Raghu, because the kids are playing hide and seek. The entire time Ravi is hiding, he is envisioning what his victory will feel and look like. Ravi stays hidden for so long that all of the kids forget about him completely. By the time he runs to the den to claim victory, all of the other kids have moved on to many other games. The kids and the parents all tell Ravi to stop acting so foolish and join in the new game if he wants to play.

Of course Ravi is supremely disappointed that nobody is willing to recognize his victory. The description of his disappointment is contained within the last paragraph.  

But he had been forgotten, left out, and he would not join them now. The ignominy of being forgotten—how could he face it? He felt his heart go heavy and ache inside him unbearably. He lay down full length on the damp grass, crushing his face into it, no longer crying, silenced by a terrible sense of his insignificance.

I get the feeling that Ravi is experiencing a mixture of anger and depression. The anger I get from the fact that he refuses to join in the new game. He's doing what my own kids do when they don't get their own way. He's pouting, which I feel is a more angry than sad response. As for the depression, I get that impression from the fact that Desai writes that Ravi is not crying, yet his heart feels "heavy." Add that to the "ache inside" of him, and I get the impression that Ravi is angry and depressed over the fact that the kids still think of him as completely insignificant. He is completely crushed by the events, because he believed that by beating Raghu, he would be cheered as a conquering hero of sorts.