A slap can be literally administered on the wrist, rather than on the face, let alone the eyes, as when you find an offending hand in the cookie jar--although I suppose cookie jars are more of an American phenomenon than English. I don't know if parents still slap their children, but obviously some naughty behaviors are worse than others, so a literal slap on the wrist is still a feasible way to punish mildly. But it is commonly used today in the figurative sense, ostensibly punishing or reprimanding, but so weakly as to vitiate any sense of forcefulness.
SS