A child may stop breathing for a number of reasons such as obstructed airway, head injury, drowning, allergic reaction, or asthma.
Obstructed airway is the most common reasons in smaller children.
If a child has stopped breathing, immediate emergency action is needed. Call 911 for an ambulance. Make sure they are not breathing. If any foreign object is visible in the mouth or throat, remove it. Pinch the child's nose shut.