I have a 6 year old son, his booster seat is mostly in my wife's car, since the third row seat is small, I was wondering if legally I need a Booster seat for him to set in the third row or not?
Searched the web, coudn't find any info.
For regular cars/seats every child age less than 8 or less than 48" tall needs a booster seat.

 

I too have small children that need to switch between vehicles. Just to be on the safe side we have child's seats in both cars. Legally you could probably put him in the 3rd row jumper seat. Though personally, I would always have them in a child's seat or booster until they are either old enough or large enough to sit in a regular seats. Third row jumpers aren't really safe in any configuration, IMHO.

 

The law has nothing to do with the seat size in the car. Its based on a height and weight of the child and here in ky it also capped at age. If the child is under 7 it does not matter if he or she is over the height and weight limit they still have to ride in a booster. Besides its not worth the risk for the trade off of a couple minutes to switch car seats around. We have 4 kids, so I know its a pain swapping seats between the cars but, my 5 and 7 year old both ride in the 3rd row and Im still making my 7 year old ride in the booster. These are state laws so they are going to vary, check your local transportation department website or give the local police/fire dpt a call and find out the details of your states laws on car seats.

 

This is from Michigan state police website. Michigan's child passenger safety law requires:

Children younger than age 4 to ride in a car seat in the rear seat if the vehicle has a rear seat. If all available rear seats are occupied by children under 4, then a child under 4 may ride in a car seat in the front seat. A child in a rear-facing car seat may only ride in the front seat if the airbag is turned off.

Children to be properly buckled in a car seat or booster seat until they are 8 years old or 4-feet-9-inches tall. Children must ride in a seat until they reach the age requirement or the height requirement, whichever comes first.