Car Seat / Booster Seat ?

The seat you're asking about is called a "Combination seat" - a forward facing harnessed seat that can later be used without the harness as a booster seat. The ones most frequently recommended by certified Child Passenger Safety Techs for how well they work as a harnessed seat (high height and weight limits to ensure that the child doesn't outgrow the harness before being developmentally ready for a booster) and as a booster (positioning the shoulder belt across the center of the shoulder and the lap belt low across the tops of the thighs and not over the soft abdominal tissue, and able to adjust tall enough to fit most kids until they can safely fit the adult seatbelt) are the Britax Frontier/Frontier85 (retail around $280), and Graco Nautilus (retail around $170). The Evenflo Maestro is a good budget option (retail around $80) but because it doesn't adjust very tall in booster mode, your child would need another booster after outgrowing it. Most other combination seats are not recommended because they are either outgrown before the child is mature enough for a booster, or they position the belt poorly as a booster, or both (go to http://www.carseatblog.com and search "combo seat review" for specifics)

Remember that each step "up" in car seats (rear facing to forward facing harnessed, forward facing harnessed to boostered, boostered to seat belt only) is a step "down" in safety so don't feel rushed to move your child to the next type of seat if they still safely fit their current type of seat. Many if not most children don't develop the impulse control and maturity to sit properly in a booster until at least age 5, some not until age 6. Many technicians would recommend that your 2.5YO remain rear facing - see http://carseatblog.com/?p=5168 for details) but if you plan to keep your 2.5YO forward facing, the Frontier or Nautilus should last until he/she fits the seat belt without a booster.

We have the Graco Nautilus and we LOVE it!! Our little one is 3 1/2 and she is still in the 5-pt harness...the 5-pt harness holds up to 65lbs, then it converts to a high back booster, then finally, the back comes off and it's a backless booster! It's the last carseat you would ever need!

Your Britax Marathon I would believe should be fine - they have I want to say a 5 or 6 year period of life where they are considered "safe". Maybe even longer. I think you're on the right track. That's what I would do.

We had to buy a combo seat (convertible & booster) because our daughter outgrew her Britax Roundabout before turning four. I'm regretting the decision. The booster with the Britax Frontier is basically a car seat where you can run a seat belt through the side. We thought it converted to a real booster seat, where we could easily transfer it from car to car. No, still the same massive size, horrible to install nightmare. If you're big on harnassing for a long time, you'll be fine with a combo seat, but we really wanted a regular booster and you don't get that with a combo. If I had to do it over again, I would have purchased a cheaper regular car seat with an extended weight range to get us through until she was four and then bought a regular booster seat.

In your case, I'd keep your oldest in her current seat and buy a new regular car seat for her younger sibling. Then buy the regular booster seats when the time comes.

The two big models in combo seats are either the Graco Nautilus or Britax Frontier.

See if a Graco Nautilus will fit the bill for your needs.

Hello my kids are now 10 and 6 for my 6 year old we went with a car seat that also worked as a booster the poundage for it is 5-100 lbs. It was the best one we ever bought. We left it in my car because it was tethered in even as a booster so i didn't have to worry about the booster leaning forward when i braked. My son loved it till his legs got to long. We also bought a half booster for my husbands truck so we didn't have to untether the big one when we went somewhere in the truck. I believe we bought it at Babies R Us. It was a Costco brand I think that was a long time ago, and if I remember correctly it only costed between 100 & 150 dollars. I really hope this helps you.

I purchased The Learning Curve First Years True Fit Car Seat for my son, it is rear facing up to 35 lbs forward facing in the 5 point harness up to 65 lbs and rated up to 80 lbs as a booster with a regular seat belt.

There have been a lot of posts about carseats recently...you should check those out along with the National Highway Traffic and Safety Association's website on guidelines/best practices.

Yes, there are convertible carseats that will later convert to a booster seat.