These personality traits were an attempt by the author to showcase his failure to understand or appreciate human life, much in the same way that many young children fail to fully understand the permanency or tragedy of death. In one line he says,
“To die will be an awfully big adventure“.
~Peter Pan clearly showing his immaturity and disconnection with the permanency of death.
Having just read how different Peter Pan was, wait until you read about Pinocchio.
This is of course our own interpretation of the text. The author, J M Barrie, never explicitly reveals what “thin them out” means. It could actually mean that he sends them packing, but it doesn’t really fit in with the darker theme of Peter’s personality.