I had this happen many years ago. It was deemed the fault of the car pulling in.
I put in my policy that if another parent is in the driveway, arriving parents are required to continue down the cul-de-sac to turn around and wait. It also helps with confidentiality and reducing backover risk to children.
We know how well some supervise on the way to the car.
I'd recommend letting them work it out. You are not at fault.