The main lullaby theme in this score is very similar to one of the songs in Disney's Jungle Book: it's the song that is sung by the girl that Mowgli follows at the end of the film. I think it's called "fetching water from the well" or something: la la la la la la laaa la , la la la la la la laaaa...
As soon as I heard the female voice humming the tune in Pan's Labyrinth I was instantly reminded of something I'd heard before only I couldn't quite place it. Well, there it is. The song that the little girl sings at the end of The Jungle Book: la la la la la la laaa la....