Tommy's probably right- the clothes will give you the most accurate timeframe, but given the probable timeframe, they may not be much more accurate than the car. If the photo was taken during the Great Depression, the clothes could be as much as ten years old (although these don't look it). The kids fashions should be the best guide because their clothes can't be more than a year or two old, but I don't know how much kids fashions changed in the 1920s to 1940s. Especially boys' clothes.