
Since pediatric patients require less energy to convert their ventricular fibrillation, or in other words- DE-fibrillate them, (see blog entry 'how exactly does an AED help', or video 'how an AED works' for background on this process), it is advantageous to use the smaller dose if possible, as there would be less tissue burning from the shock itself.
If no pediatric pads are available, adult pads can still be used, because the ultimate priority is to stop the ventricular fibrillation and save the patient's life, and a higher dose will do that as well.
Perhaps counter-intuitively, pediatric pads should never be used on adult patients, as the relatively small dose can actually make the ventricular fibrillation worse, and even harder to correct.