Arizona State Law requires children to be in a child restraint system (car seat) until their fifth birthday. There is no longer a weight exception. A booster seat is also required for a child who is at least five years old but under the age of eight and who is not more than four feet nine inches tall.
Seats in cars are designed for use by adults so consequently little people do not fit properly. The lap belt should fit snuggly across the hips not the stomach. In a collision a lap belt across the stomach will put too much pressure on internal organs which will result in injury.
The shoulder belt should fit over the shoulder and across the chest. At no time should the shoulder belt cross the neck. Many children will place the shoulder harness behind there back to avoid it rubbing on the neck. This is like not wearing a seat belt at all. The seat belt was not designed to be worn like this and will not provide adequate protection in a collision.
There are many items that are available to make the seat belt fit the child. These include extra belts and clips that attach to the seat belt itself to squeeze it down to fit the child. Most of these items are not crash tested so it is unknown how they would perform in a collision. The only safe way is to make the child fit the seat belt. This is done by using a booster seat. This makes the child sit taller so the seat belt will fit correctly across their body.
Many people ask which is the best child seat available. The answer is to get the one that will best fit your car, your child, your budget, and one that you will use every time your child gets in the car.