just a quick run down, children should be rear facing at least till they are 2, but really till they reach the max rear facing limit on their convertible carseat (provided they have 1 inch of shell over their head). most convertible seats RF to 33 or 35 pounds.
then once forward facing they should remain in a 5 point harness till they're around the age of 6. children under the age of 6 are not able to control their impulses which means they are not going to sit properly in a booster. of course if they still fit in their harnessed seat then they should continue using that until they reach the limit since a 5 point harness is safer. (4 years in the MININUM for a booster). most seats use to only harness to 40# but there are many seats out there that harness past that (50/65/even to 80#).
then its time for a booster. the booster seat helps to properly position the adult seat belt on a non adult body. they should remain in a booster until they pass the 5 step test
- Does the child sit all the way back against the auto seat?
- Do the child's knees bend comfortably at the edge of the auto seat?
- Does the belt cross the shoulder between the neck and arm?
- Is the lap belt as low as possible, touching the thighs?
- Can the child stay seated like this for the whole trip?
and above all the seat needs to be properly installed and properly used. the harness should always be snug and the chest clip should be at chest level.