This is a delightfully dark opening scene.
The children are shot from above -- perhaps to underscore how small and vulnerable they are? They waste no time introducing the idea of a murderer to the audience -- by having the girl sing a song, it immediately connects in our minds the murderer to the children, that they are the ones in peril here.
As someone else pointed out, there is no sound here that doesn't exist within the world of the movie, whether it is the girl singing, the woman scolding, or the horns honking. The silence is effective at creating a forboding feeling in the view