Do we need to use the plural of fruits here? I don't see why "La femme a un peu de fruit" should be wrong, e.g., if a woman just carries half an apple or something like that.
"a little bit of" = "un petit morceau de" or "un petit peu de".
If you are referring to a woman selling "un peu de fruits" on the market, it means that overall the quantity is small yet several different fruits are displayed.