Vehicle in motion- Children are energetic and tend to play inside cars when they are bored. This can cause them to accidentally set the car in motion and find themselves in the middle of a busy road.
Power accessories- Having electric devices and heavy object inside the car with children can be a potential hazard to their health and safety.
Car crashes- Even if your car is not in motion children should not be inside if not necessary, another driver could collide into your vehicle harming the passengers who may not even be wearing a seat belt.
Kidnapping- Not very common but occasionally unattended children have been know to be taken from a vehicle.