There is "chinko", "chin chin", "chinpo", "chinpoko" and "penisu". The last one would be used more for serious situations (in a hospital for example), whereas the first three are used interchangeably. "chinpoko" sounds more juvenile. "chinko" and "chin chin" are, I strongly believe, interchangeable. Even though if you look at the dictionary it would be written juvenile word for "chin chin". So feel free to use any, for jokes I recommend you "chinpoko".