Myles is too big for his car seat, and I purchased a full back booster seat that goes from 30-80 lbs, and can be converted into a backless later on. It was okay-ed by our ped, even though he isn't four yet, because he is big for his age (height and weight).
My MIL got a backless one for him, and would like to put him in that when he is in her car. I always thought they had to be in a full back booster seat until 60lbs (or I thought that is what I read anyway), so I told her no. She got pissy with me and looked at me like I was crazy. Now I am researching it online, and I can't really find a concrete answer about when to switch to backless, or if you can go straight to backless.
Do any of you know anything about this? Or know a website I can look at? I am mostly interested in what is safest, not just the laws.