The idea of a helper robot does not need to be strictly related to ice cream, nor does it necessarily need to use Bluetooth to accomplish its goals. We've made a few diagrams showing different ways to set up a helper bot scenario.
The above diagram shows a robot that uses sound to communicate between NXT bricks. A different level of frequency recognition could differentiate between stations.
This diagram shows a procession of "follow the leader" robots that would all move to a specified area in turn, and accomplish their specific task. Depending on the number of NXT bricks involved, this could be done using timers, sound, or Bluetooth.
Finally, this sketch shows one of the more challenging setups for a helper bot. Using line following, it would be possible to set up each NXT with a touch sensor and use that as a method of communication.