you are right, your kids are not big enough for a booster seat with the adult belt! Not only are they under 3yrs old, hte BARE MINIMUM for a booster seat, but A child under 40lbs (and about 5/6yrs old) is at risk for submarining under the seat belt in a crash, and at a very high risk for seatbelt syndrome. Google seatbelt syndrome for more information, and be prepared to gasp in horror at what happens to a child who is not big enough to be in the adult belt.
Anyway, it seems you are already following your instincts that your children are way too small for a booster seat. You have a few options...
Graco Nautilus, harness to 65lbs, harness fits most kids to age 7 height wise as well, and then becomes a high back booster to 100lbs, and a low back booster if necessary. The no back booster has a 9yr life span, so its a great deal for $150, and you can find it for cheaper online sometimes, and I hear there are going to be some good deals on car seats tomorrow online. If you use the top tether (which is recommended anyway), this seat performed well in crash tests.
Britax Regent- harness to 80lbs and fits most kids to age 8+ or 4'9" when they can sit in the adult belt safely. This is a HUGE car seat, we have one and we LOVE it. Our 7yr old still fits in it perfectly! You can find amazing deals on this seat right now as they are being discontinued, so yo ucan find them online at most retailers (Target, Wal Mart, Albeebaby, etc) for around $150. This is a smokin' deal as the Regent retails for $300! Top tether is required for use in this seat over 40lbs.
Britax Frontier- Harness to 80lbs, becomes a high back booster. Makes a great booster too, it fits children very well. This is a great seat that will last, has great side impact protection. Its around $270. Top tether required in this seat as well.
Evenflo Generations 65- harness to 65lbs then becomes a high back booster. this is just a revamped version of the older Generations car seat with a higher weigh limit, so if you are interesetd in this seat ,try it out at Babies RUs first to make sure your girls have room to grow in it height wise.. Its $99 at BRU.
Safety 1st Apex 65- harness to 65lbs, high back booster to 80lbs. CAn be a pain to use because it requires high seat backs or adjustable head rests to use. But its a nice seat for the price (can be found as cheap as $80 online at albeebaby.com) and it makes a decent booster.
A car seat is outgrown forward facing when the child goes over the weight limit of the seat, the shoulders go over the top harness slot, or the ears become level with the seat back. When one of these things occurs, the seat is outgrown and you must stop using it.
Good for you for knowing your car seats are expiring (6yrs from date of manufacture) and that your children are too small for the adult belt booster seats. Any one of the seats I listed above would be a great fit for your children until they are mature enough both physically and mentally for a booster seat.