The main benefit of the parent and child spaces is supposed to be the additional space either side, which I suspect is useful if you have smaller children who need to be strapped to a car seat etc. Once you no longer need the additional space, you shouldn't use them (probably about 6 years old for most kids). I suspect parent and child spaces would be more useful for their actual purpose if they were situated further from the door, which would discourage their general use by people who don't need the side space.
I don't really understand the 'special spaces by the doors' thing. Yes, disabled spaces should be by the door, for obvious reasons, but parent and child spaces don't need to be. Personally, I'm a lazier parker than walker, so I'll park in a drive-though space at the other end of the car park and walk across rather than spend 5 minutes parking in a space where I can't open the doors once I'm in.