Movie is from the 50's? There is a man looking where money has been hid by his partner. The partner died and the man looking for the money found the dead mans wife and two children a boy and a girl. The children hide and the bad guy kills the woman, a blond and puts her and her car in a lake. The children then get into a canoe and sail away to a house where an old woman takes them in. Lots of whistling in this movie.

The Night of the Hunter (http://www.tcm.com/tcmdb/title/17678/The-Night-of-the-Hunter/)