Other mistake: During the time capsule recovery ceremony, there is a man standing next to Nicolas Cage, who pulls out his cell phone to take a picture of the ceremony. However, he's holding the cell phone wrong, pointing the camera towards himself.
Continuity mistake: When John is decoding the numbers for the first time, he uses a whiteboard. On the whiteboard are, presumably, spelling words for Caleb. He erases some of these to make room for the numbers. In the various shots of this scene, words disappear and reappear continuously, often written slightly different.
Answer: There is no intention on the part of the whisper people to warn humanity as a whole, merely a select group of children who appear, due to their telepathic powers, to be the next stage in human evolution, and their respective guardians.