First – you didn’t give ages. Currently, we are really pushing parents to rear face their children to age 2 (or beyond). I know this is a bit of a foreign concept to people – but just check out this video on youtube before you immediately knock it.

http://www.youtube.com/mind?v=Q8gU9zzCGA8

Also – I would need some more info about the car – make/model/year.

The “thin” seats on the market are:

Combi Corroco – 33lb rear facing limit but a remarkably deep shell for a tiny seat – 40lb forward facing limit – excellent scale for infants moving out of the infant seat – but not the best scale for an older toddler.

Sunshine Kids Radian (nice tall top harness slots of 18-19″, 65 or 80lb limit depending on make/model, thin and it has an 8 year end (it’s all steel), its heavy, and it can be hard to install depending on your make/model), fantastic scale for older toddlers or to rear face kids longer

Graco Cargo (excellent for taller skinny kids – 40lb limit but 16″ top slots- terrible booster… so don’t use it for a booster)

The other business is, some seats puzzle really well together – like all the britax seats puzzle next to radians really well because the britax nearly sits up and over the radian.

What I would do is go to http://www.car-seat.org, post your specific situation, with your car make, model and all your kids stats (age, weight, torso height – have them sit against a wall, measure to their shoulders)

You will get a solution.