The shed appears to be a great place for Ravi to hide from Raghu for two reasons:
1)The shed is dirty and manifests an atmosphere most children would characterize as spooky. Furthermore, rats, spiders, and white ants are in the habit of dwelling in the dark shed, as Ravi mentions. The shed also houses broken bits of furniture and defunct household items. Because the shed is such an unattractive hiding place, Ravi reasons that no one will look for him there.
2)With the shed locked, only a very small person can slip through the small gap between the hinges of the door and the wall. Since the gap is only small enough to allow rats, little dogs or small children through, Ravi manages to evade Raghu's presence. Ravi is exultant that he has probably discovered a fail-safe hiding place, as his older brother would probably not be able to fit through the gap.