No, they do not. That's why it is actually safer to keep children in a 5 pt. harness carseat as long as possible.
The MINIMUM requirenment for booster use is 4 yrs. AND 40 lbs. BUT the safest is to keep them in a 5 pt. harness.
Here is more info:
http://homepage.mac.com/Y./FileSharing32.html
Some videos:
http://www.youtube.com/watch?v=azgBhZfcqaQ
http://www.youtube.com/watch?v=NxfIuhL3cXk
If you need more help go ahead and send me a msg. w/ your children's age, weight and torso height. To measure their torso just sit them on the floor against the wall w/ their feet straight in front of them. Then measure from the floor to just above their shoulders NOT their heads. Let me know what carseats they are in at the moment and what car are they installed in.
INFO FOR THOSE W? BIGGER KIDDOS:
The carseats that convert into boosters are called combination carseats. The best one in the market right now is the Graco Nautilus. It has a 65 lbs. limit for the 5 pt. harness, it has 18 in. top slots and converts into a high back booster and a backless booster up to 100 lbs.
It is only $150 and can be found at Walmart, online and someone said at Target too.
http://www.walmart.com/catalog/product.do?product_id=8112222
There is also the Britax Regent which doesn't convert into a booster but it is a great seat because it has an 80 lbs. weight limit and 20 in. top slots.
For more info or question you should visit www.car-seat.org and the techs there will help you.
Response to other posters:
Nicole B: NOOOOOOOO lol don't do anything that changes the way a carseat works UNLESS that manual states that you can. Booster seats can only be used w/ the lap/shoulder combination seatbelts. They should never be used w/ a lapt only seatbelt.
FOUND THIS OUT ABOUT BOOSTERS THAT ATTACH TO THE CAR:
Jane Indy Plus, about $200.
Sunshine Kids Monterey, about $130, due out in the next few months.
Britax Frontier combination seat, about $300, due out in the next few months.