blimp:
Sylvia's mother says, 'Please don't say nothin'
To make her start cryin' and stay'
Now Arthur, if you're going to quote from the song "Sylvia's Mother," by Doctor Hook and the Medicine Show, for our younger Library members (those who grew up with cell phones) you should explain what an "operator" is and why she keeps asking the desperate singer/narrator for more money.
As for F/g stories with outside non-family witnesses, it's easy enough to employ the "Search" function to find stories featuring both spanker and spankee being females, with characters who are adult and child in terms of age, which in the vast majority of cases would involve a woman spanking a girl. You would have to read the synopses of the stories found for indications about witnesses, I'm figuring.
In the "Sylvia's Mother" stories, it strikes me that there's an unstated (and possibly subconscious) understanding among the mother, the boyish witness (Philip) and Sylvia herself, which is why the girl's maternal spankings end up being observed by her preteen neighbor so often...

--C.K.