The beginning of M clearly sets the stage for what is to come. I was struck not only by the song that the children were singing, but the fact that (at least for the first few moments of the film) while they were singing it they are entirely alone. In fact for much of this opening adults are at a removed distant if not entirely absent. The only adults around are either high above their children, as is the case with the two mothers, or come about only after the children have narrowly avoided danger as in the case of the policeman. As someone else pointed out above, most of the adults in the opening seem to be not even paying attention to the children around them. Lang does a terrific job of creating a feeling that the children are at risk simply by removing a few elements. Adults are too far away to help and we can feel how exposed the children are.
Lang uses silence in this opening too, the effect of which is that when there are sounds (clocks, bells, car horns, etc) they are even more jarring and menacing. Interestingly many of the sound effects used in the opening are sounds that are used to get someone's attention or alert them. The clock chimes to alert us to the time, bells toll to alert us to events, car horns honk to alert us to danger or approaching danger. Perhaps this was Lang's way of continuing to put us on edge by using sounds that our subconscious would recognize as signals to pay attention
I was struck too by the use of light and shadow. Even though the scene takes place during a sunny day there is a sense of menace. The shadows are long and pointed and even the cuckoo clocks looks devilish. And this is what I think makes this opening even more frightening...it is a lovely sunny day and yet something is wrong, children are being stalked by a man in black. This isn't come murderer stalking back alleys at the dead of night, this is a man who can walk about in the daylight and talk kindly to little girls on the corner with no one any the wiser.