Final answer:
If you witness the collapse of a child and confirm no breathing or pulse, immediately begin CPR by performing chest compressions and rescue breaths.
Step-by-step explanation:
If you witness the collapse of a child and confirm that there is no breathing and no pulse, your next step should be to start CPR immediately. CPR, or cardiopulmonary resuscitation, is a life-saving technique that involves chest compressions and rescue breaths to maintain blood circulation and provide oxygen to the body. By pushing on the sternum in the correct position and at the appropriate depth and rate, you can manually compress the heart and help circulate blood.