Poison Ivy made a "Kelly Carlisle" to serve as her son. The actual Kelly, however, was a girl. Kelly was actually a plant-based lifeform enhanced with Doctor Carlisle's genetic material. He was apparently programmed with whatever skills and knowledge he needed to function in the world and pass as a child without arousing suspicion.
Kelly and his "brother" Chris were excited to find the Batmobile parked outside their house, and were stunned to find Batman himself inside. Although they wanted to get a ride with him, Steve sent them off so the grown-ups could talk.