What's the name of this movie?

It's a movie about a woman who moves into a big fancy house, I think somewhere in the U.K and two children live there a boy and a girl cause their parents died and they live with a nanny I'm guessing, who's rude to the woman at first... Mmmm thats all I can remember and at the end the children drown in a pond and the lady goes crazy and there's like a bright light of some sort. It came out a few years back and I cannot remember the name and it's driving me nuts, please help (: