A player of a board or card game that acts immaturely. E.G. Tries to take back moves, Rubs winning in, Berates themselves or others for losing.
Kiddie-pool player: FUCK, I CAN'T BELIEVE I MISSED THAT. WHY AM I SUCH A RETARD?!?
Player 1:Go play in the Kiddie-pool.
Player 2: Yeah, you're being a real kiddie-pool player.