After you talk to all the spies in the level and return to get the shovel, go back into the shadow world and switch the shovel for another Pegasus Boot or the empty item stand. Leave the shadow world then the area with another item other than Pegasus Boots and go back down into the Dark World. The passage to the shovel will be blocked and the kid won't open it again. Talk to him and he will give you Pegasus Boots and you'll drop the other item. Keep picking up the other item and talk to the kid again for an infinite supply of the Pegasus Boot.