A. You probably shouldn't, as it may not be the safest thing to do. There are belt-positioning booster seats with minimum weight limits of 30 to 33 pounds, but according to the American Academy of Pediatrics 'your child should stay in a car safety seat with a harness as long as possible before being allowed to ride in a booster seat.' And since there are many convertible seats and combination seats that allow you to use harness straps up until your child is 40 pounds or more (the Britax Marathon can be used up to 65 pounds with a harness), you could keep your child in a car seat with a harness strap a little longer with one of these seats.
The National Highway Traffic Safety Administration also states that 'your child should stay in a car safety seat with a full harness until the seat is outgrown, usually at about 40 pounds.'
Of course, she would also have to meet the car seat's height limits for using the harness straps, which is usually about 40 to 43 inches (49 inches for the Britax Marathon).
Why do they sell booster seats without harness straps with weight limits below 40 pounds if you should use a harness strap below this weight? Mainly because some taller kids outgrow their car seat even before they reach 40 pounds. So if your child's shoulders are above the top set of strap slots on your car seat, then you can move him to a belt positioning booster seat, or simply stop using the harness straps if you have a Combination seat, and now use the car seat as a belt-positioning booster.
Car Seats with High Harness Limits
The following car seats have high weight and height limits (65 pounds and 49 inches), so you can keep your child in them using harness straps 'as long as possible':- Britax Boulevard
- Britax Decathlon
- Britax Marathon
- Safety 1st Apex 65
- Sunshine Kids Radian Car Seat
Sources:
AAP. Car Safety Seats: A Guide for Families - 2009
NHTSA. How should preschool and school children ride safely?
