Booster Car Seats...

I would not go with a booster.
Please watch this video, and/or search "5 point car seat" on youtube and watch many many videos on the dangers of booster seats.
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I would never put a 3yr old in a booster regardless of size! My almost 5yr old is 45lb and 43" tall and is still in a harness, because regardless of your childs size, be it a 50lb 3yr old or a 20lb 3yr old, they are still internally a 3yr old and the damage will be done... physical size doesnt' matter, internally the child is still their chronological age. Higher harness seats are now as low as $80, no reason for a 3yr old to be in a belted, let alone a backless, booster.

Ok rant over. I would move your baby to the Roundabout rear facing, and leave her that way until she outgrows it (33lbs, or less than one inch of hard shell above her head.) Google EXTENDED REAR FACING and JOELS JOURNEY for more information on why you want your children rear facing as long as possible.

Then move your oldest to a higher harness car seat like the Graco Nautilus or the Britax Frontier. Either is an excellent choice for his age/height/weight.

The Boulevard is an excellent seat, but your older child would benefit from a new higher harness seat for a larger older child, and your baby would benefit from being rear facing in the Boulevard.

As long as you are going to use the Frontier's 5 point harness for your 3 year old then I say its okay to get that one. We have the graco nautilus. I'm not familiar with the boulvard will your 1 year old still be able to rear face? They recommend leaving your child rear facing until atleast 2 now. If you can't then leave the 3 year old in it and get a convertible that will allow your baby to still rear face. As far as putting them in a booster for the convenience of them being able to buckle and unbuckle themselves my 3 year old can climb into his nautilus and buckle it he needs assistance with the bottom buckle when getting out but his brother usually does it for him I would not put a child in a booster using a seatbelt before the age of 5 but that's just me.

To be in a booster seat a child must weigh 40 pounds.

I highly suggest you go with the Frontier 85. We have 2! (One for our 5 year old and one for our 3 year old.) We all love them! And once they hit 85 pounds or the height restriction (pretty tall-can't remember exactly off the top of my head) then you can use as a booster till 100 pounds. And the biggest bonus-----NO THREAD HARNESS! So easy to adjust. And my husband gets a tight install in less than 5 minutes.

We got ours on sale for $223 at albeebaby.com

Safety first. We will not booster our kids until they outgrow the 5 point harness on the frontier 85. A friend of mine has her just over 3 year old daughter in a booster and she looks like she will fly out of it if they would be in an accident. She can twist around and practically get out of it when buckled! A booster seat in itself is NOT safe for kids this age. We wouldn't even trust our 5 year old in one.........

Check the height and weight requirements and recommendations for the car seats and booster seats. My three year old is tiny and is a long way off from being able to ride safely in a booster. You could also call your local fire department and they can give you advice - they are a great resource for car seat safety information!

For the safety of your oldest child I would buy your baby a convertible car seat, and leave him/her rear facing for as long as the new seat allows. Leave your oldest in a 5 point harness until he/she out grow it, that is the safest place for he/she.

After my son turned 1 I took him from the infant carrier to a rear facing convertable seat, I was in a car accident a year later and the seat had to be replaced. This time I replaced it with a froward facing seat (( Graco Nautalis)) that had the 5 point harness that goes up to 65lbs and converts to a belt positioner and then a booster with a back or w/o ((depending on your child)) -- I personally would go back and make sure this is the seat I would have bought from the get go after my son reached the limit on the infant carrier seat. This seat is made of steel (( downfall-- heavier)) vs plastic -- however much safer in an accident, and the harness lasts much longer then the usual "40 lb" weight limit of the reversible car seats. My son is on the taller side and is at 33 lbs now and will be 3 in a few weeks, I want him to be in a 5 pt harness as long as possible (( he has quite awhile before outgrowing the weight limit ))

My daughter was moved from the britax marathon to the britax frontier at age 3.5 because, like you, our baby needed the marathon! Yes, be sure to check the minimum weight on the frontier! It is heavy but sturdy, and we all love it! Safe, comfortable, trusted company, durable. They can stay harnessed to 85lbs, which is the highest of all 5point boosters! This keeps them safest for a long time. There are good deals at albeebaby.com, and I've seen new in box frontiers on eBay with free shipping too. Good choice!

We just put our 3 year old son in a backless booster from Cosco ($17 at Target). Gasp! He's only 3, how dare I? My son is 42" and weighs 43 pounds. He's bigger than his 5 year old cousin and he wears 5T clothing. So he is the size of a 5 year old. We also did this because he outgrew his 5 point harness and my daughter is about to turn 1 and is too big for her infant carrier. So now she is in her brother's old convertible carseat, rear-facing of course. And her giant brother is in his new booster, which he loves. Back to school time is the best time to get deals on everything. I would search Ebay and discount sites on the web to get the best deal on whatever seat you decide on. Good luck!

I put mine in around 3 they are big enough then. And they can unbuckle themselves which I find helpful.
J.