If there are plenty of spaces available then I don't really care if others park in them. Like later at night there are 20 child spaces sitting open at supermarkets, they're obviously not all needed. However, able-bodied adults taking the LAST space is a dick move. I'd be OK with a blue badge holder taking it though, if all the disabled spaces were taken.
That said, have you looked at the layout at some supermarkets? At our local Asda, half of the parent and child spaces are FURTHER from the door than some of the normal spaces
. Bizarre.