How do you know when your child is too big for their forward facing harness seat? The seat is outgrown when ANY ONE of the following happen:
1- His shoulders are above the top harness slots in the seat
2- The tops of his ears are above the top of the carseat
3- When he has reached the maximum weight for the harness (usually 40, 50, or 65)
So what do you do now? Is your child ready to use the vehicle seatbelt? NO! Children are not usually ready for a vehicle seatbelt until 10-12 years old. So, what do you do? If your child is at least 4 years and 40 pounds, you buy a booster seat. If your child is not 4 years old, or 40 pounds, he is really too small or to young to use a booster, and should be in a high weight harness.
Booster seats come in a few varieties.
1- High Back booster- This booster has a back on it, which may be removable
2- Backless booster (or no back booster)- this booster has no back, only a seat and arms/handles 3- Combination seat- this is a seat that is a forward facing harness seat, and a belt positioning booster
Which type of booster is best?
The one that fits your child and fits your car and fits your budget :)
I choose to use a high back booster for the children I take care of. A HBB povides more side impact protection. With a HBB, your child's head will hit the "wings" of the booster, not the vehicle door or window.
Also, please note that children don't usually have the maturity to sit properly in a booster seat until 5-6 years old. If they can not sit properly the entire ride, they need to be in a 5 point high weight harness seat. That will allow you to use the harness to 65 or 80 pounds (also depending on the child's height). The 5 year old I take care of still rides in a seat with a harness.
Here are some crash test vidoes to watch before selecting your booster seat.
High back booster with deep side wings:
High Back booster with shallow side wings: