I am planning on using a dvd player without headphones on our flight to Australia this summer ()
imo, if the sound is not blaring out, it is only the same level as having a conversation, and no-one would think twice about that...
I would second using your dd's cabin baggage allowance and cram a bag with toys/books etc.
we got through an 11 hour flight with the dds last year (then 3 and 1) using the following:
food/snacks - always a winner
books - a mix of old favourites to reassure, and new exciting ones
those magnetic doodle screens - drawing without mess!
LOTS of talking and singing songs
but dd1 (autistic) was not interested in drawing or tv or jigsaws, so that was a big drawback for us.
will do similar this year, but with the added bonus of Mr Tumble on the dvd player
would your dd like those small wind up toys? you could spend a while racing them across the table? (and they come in all shapes and sizes, so could pack a few to keep it interesting)