Orson Scott Card is not averse to dropping symbolism in his books. Ender's Game is loaded with symbolism, and the part that this question asks about is no exception. Ender is playing the game with the Giant that he had killed earlier, and this time Ender goes past the decaying Giant into a forest. After the forest, he comes across a playground.
He went around the giant's corpse and followed the brook upstream, to where it emerged from the forest. There was a playground there, slides and monkeybars, teeter-totters and merry-gorounds, with a dozen children laughing as they played.
Ender then notices something. His avatar is now a child, and he is a smaller child than all of the other children present. He also notices that he is not capable of playing on any of the playground equipment. His avatar simply falls off or through each thing. The other kids laugh and tease Ender, and he decides to leave. The other children turn into weird wolf creature things that tear Ender apart, but Ender's avatar reappears each time until he figures out a way to kill all of them. Ender essentially tricks the creatures into falling off of the playground toys as he falls through. This stuns the creatures enough for him to drag them into a creek to be killed.
This time, as soon as the child hit the ground and turned into a wolf, Ender dragged the body to the brook and pulled it in. Each time, the body sizzled as though the water were acid; the wolf was consumed, and a dark cloud of smoke arose and drifted away.
The symbolism here is essentially affirming to Ender and to readers what we already know. Ender might be one of the younger and smaller children in Battle School, but he is being asked to behave in a very adult-like manner. He is being tested in ways that no child should be tested, and that is forcing him to grow up very quickly. He is not a kid that is being allowed to play on the playground like the other kids. Ender is a boy that is not being allowed to develop at Battle School at the same pace the other kids were allowed. The other kids recognize this difference about Ender, and most of them hate him for it. Ender knows this too, and Ender knows that in order to survive, he is going to have to beat the other students at their own game. Just like he beat the wolf things by using the playground equipment, Ender is going to have to keep finding unique uses of the Battle Room.