For the transmitter, you need an oscillator , running at the transducer frequency and handling athe correct amount of power. For the receiver, you need an amplifier to increase the level of the received signal followed by some sort of detector.
If robot B receives a ping , how does it know when it was sent? . One way would be to send out with robots A ping, a flash of light/infra red. As light travels much, much faster then sound, then the reception of the flash could start robots times which is stopped when it gets the supersonic ping.
Frank