Anything fleece lined, rather than down-filled, is a pretty safe choice, usually. Like the Lands' End squall jackets are usually pretty good.
Also, you can unzip the coat and put the straps right on the shoulders so kiddo is still warm, but the straps aren't compressing a lot of extra fabric. Some people take coats off and put them on the kiddo backwards after they are buckled in (a great idea for a longer trip, so they can take the coat off when the car's all warm!).
