S.C.
There are a few booster seats with the backs that still protect them from a left and right collision as well that are for children 30 lbs or more. My son who will be 5 in Sept. is very small as well (he is still catching up from being a preemie!). The only catch is that they have to be 38 or so inches. My son was right at the cusp, but I bought it anyway because the box said that if the seat belt fit in the right place then it was okay. I think he was like a half inch shorter than what he was suppose to be. These car seats are like $40 at Target (which is where I got mine). Look carefully at the weight and height as some are meant for bigger kids.