Bart was still a little awestruck that Barry seemed to actually believe him now- but he broke into a grin and nodded.
So they were jogging behind Batman... but Bart didn't really do "jogging" or "behind" very well. He beamed at his grandfather before breaking into a full run to pull up along side the Batmobile.
Kid Flash cantered along beside the driver's side of the Batmobile, turning to look into the driver's side window (or tried through all the tint). He cocked his head to one side, curiously considering the side of the car.
"Meep-Meep!" Bart chirped before putting on an extra burst of speed and pulling past the Batmobile.