I doubt that his van hit the little girl, but let's say that it is remotely possible. Then you would have to explain why there was no blood on the ground at the scene of this hit. The van would likely show some signs of hitting her as well... blood, a small dent, a scratch. Those would be the first things LE looked for. And why wouldn't the other little girl say that April had been hit, rather than she climbed into the van on her own? He wouldn't have had time to coach her what to say, would he?
The van would have had to hit her pretty hard to kill her, so if she was only injured, why would he have stopped and picked her up to dispose of her body? Why not take her straight to the hospital? Or knock on the door and tell her mom that there had been an accident? And more importantly, how did he know she was dead, he's not a doctor.
No offense intended but if one formulates a theory, one should have some facts to back it up. What facts do we know that would support this theory of April being hit, other than pure speculation?