Firecracker
As we're getting into the car on the 4th, we hear a bang:
Joe: What's that noise?
Daddy: A firecracker?
Joe: A firecrapper?
Daddy: No, it's a fire-CRACKER
Joe: What's a firecrapper?
Daddy: It's not a firecrapper. Cracker!
Update (7:11PM): We figured out Josiah was trying to say: FireCLAPPER.