Some children can twist out of harnesses at an early age. Others can easily undo the buckle mechanism. Children may also be able to put too much slack in a seatbelt, and some children don't want to use a child restraint or seatbelt at all. Driving is a dangerous activity, and like any other, it requires proper supervision at all times.
Parents can try many things. You can try calling the manufacturer of your carseat to see if they have a buckle that requires more force to release, or a 2-piece chest clip instead of a "paperclip-style" chest clip. Using a seatbelt yourself sets a good example. Refusing to put the car in motion unless the child stays in their restraint is another. Also, try to keep your child entertained or distracted if possible, and NEVER make an exception and allow them not to use their child restraint. For severe cases, you can try a safety vest by E-Z-On (800-323-6598) that may be more difficult to remove.