Question

You may not operate a vehicle unless all child passengers __________ are properly restrained with an appropriate safety device.

Answer 1

You may not  operate a vehicle unless all child passengers FOUR YEARS OF  AGE AND UNDER are properly restrained with an appropriate safety device.
Traffic law requires that children four years and below must be properly restrained in the vehicle using appropriate devices such as car seats. 
Children older than four years have to use the safety belt when they are inside the car.