Complex sentence: He is the inspiration for the comic strip Dennis Ketcham, created by his father Hank Ketcham, Dennis was only 4 years old when he refused to take a nap and somehow messed up his whole room.
Split into simpler sentences: He is the inspiration for the comic strip Dennis Ketcham, created by his father Hank Ketcham. When Dennis was four years old, when he once refused to take a nap and somehow messed up his whole room.