Final answer:
The correct response in a situation where a 12-year-old child collapses and is in cardiac arrest is to immediately begin CPR and use an AED if available, continuing until help arrives.
Step-by-step explanation:
When witnessing a 12-year-old child suddenly collapse and confirming they are in cardiac arrest, immediate action is required. The best course of action is to begin CPR (cardiopulmonary resuscitation) as this can maintain circulation to vital organs, especially the brain. If an automated external defibrillator (AED) is available nearby, it should be used as soon as possible. AEDs can analyze the heart's rhythm and provide an electrical shock to try to restart normal heart rhythm. In the scenario where you are alone and without a phone, start CPR immediately and continue until someone arrives who can help call emergency services or take over CPR.