Personally I'd be very bothered if someone left her kids including a screaming baby in the car on a hot day, never mind where she parked. Surely that would be the main concern here!
Also, why the hell would you slam your door on someone else's car? Sure, it's ignorant that they parked over the line, but you've probably cost them hundreds of pounds. What do you want, a pat on the back?
Eta- if she was only at the chemist right by the car park and had just 'ducked in' surely it's natural that she would park the closest she could? Because as you say 4 kids is not easy to haul out the car just to pick up a prescription from a shop within full view of the car.