AlanBarr:
Hayley Mills appears, along with her father John, in one of my favourite films of all time, "The Family Way". No spanking. but a truly wonderful film nevertheless (and there is a quick flash of her bare bottom at one point).
A more mature Hayley Mills showed her bare bum again in the movie, "Deadly Strangers" (1975).
In both that film and "The Family Way" (1966) it's clear that she is not using a body double.
At 74 years of age she's still looking good although I suspect that the pert derriere that she was happy to display in the past might not be quite as firm these days.