Right, for a start, you were IN the doorway surely (well, behind the door)? Strapping your lad in. You were injured, yes? As it says in the original post, regardless of how minor. Did you claim anything for that? Did you take any pictures or anything?
Regardless, the fact you were behind the door means you couldn't have "just opened it right as the 3rd party passed", as if you were exiting out the rear door. Well, you could have, but then you'd be in the road anyway, which means either way, the third party wasn't giving you enough room, since you were on the road. There is a child seat there, do you have any clear pictures of that from the incident? While a child could have potentially opened the door, there are child locks in place for a reason. Do you have a receipt for the meal still (credit card slip or anything with the time?). That would show you were leaving the area, rather than arriving, so why would you be exiting out of the back door in that case?