Some cars are not child friendly-
Convertibles, small coupes and smaller pickups tend not to be very child friendly. These types of vehicles also tend to provide less protection in crashes. Even so, it is possible for a child to ride safely in them.
First, it is never recommended that children 12 or under ride in the front seat.
Second, children should always be seated in an appropriate child restraint that is installed properly. Kids should be in a properly installed rear-facing carseat until they are over a year and over 20 pounds, preferably longer. Then they should be in a front-facing model with a 5-point harness to at least 40 pounds. Finally, kids should be in a booster with a lap AND shoulder belt until around 8 years old unless they are already 4'9" tall. The back seats of many smaller vehicles can often accomodate kids safely with some effort.
If you can't install their carseat/booster properly in back, then it may indeed be unsafe for them to ride in the vehicle. I've installed carseats in various small vehicles without a problem, but there are cases that are simply incompatible.
Please ask if you have any more questions!
Darren