Right because at that point you have to assume the intention is to kill and you likely already know there are children in the parade. You also don't know if the car has a bomb.It's not an ideal situation, but when you've got a potentially homicidal driver, you've got to think about stopping them from killing people too.
I'd probably take the shot if my line of sight was clear and I could hit the vehicle. It's always a risk though anytime you pull the trigger than an innocent person could be injured.