Sirius noted with despair that Remus shook his head negatively, indicating that he had not taken his potion as he should have. Which meant that he would not be in command of the situation once the transformation was complete. Pettigrew, too, realized what was going on, for hadn't he seen this very thing many many times in the past, and his eye was fixed upon the wand held in Remus' trembling, lengthening fist.
Sirius cried out to the children urgently, "Run. All of you. Now...."
But none of them moved, transfixed by Remus' transformation.