The fun-loving new parents laughed as they put on the black 3D glasses to watch the movie.
Kate Middleton, 31, who has been Patron of the museum since April 2013, wore a stunning black lace gown by Alice Temperley for the evening.
The thrifty fashionista wore the same dress to the royal premiere of War Horse last January.
The slinky gown, with features black lace detailing over a nude overlay, showed off the new mum's slim figure. She kept her jewellery to a minimum with a diamond bracelet adorning her slender wrist.
The new parents met Sir David Attenborough and other attendees before watching the film, which is due to be broadcast on Sky next year.
The ground-breaking movie sees TV naturalist David bringing to life his favourite extinct creatures through 3D technology. New mum Kate became Patron of the London museum in April.
The couple visited South Africa House in Trafalgar Square earlier in the day to pay their respects to Nelson Mandela.
They signed the condolence book for the former South African president who died last Thursday.
The Duke and Duchess were at the royal premiere of the upcoming film Mandela: A Long Walk to Freedom, a biopic of the revered statesman's life, at the time his death was announced.