Maybe this from SafetyBeltSafe USA is helpfull >
http://www.carseat.org/Resources/635.pdf
& this from the same site....
"Which child safety seat is escape-proof?
Unfortunately, there is no escape-proof safety seat. Children who learn how to get out of one kind of safety seat soon learn how to escape from others. However, most children quickly respond to parental firmness. This finding is based on a study conducted by SafetyBeltSafe U.S.A. with data collected from a range of English- and Spanish-speaking families.
For the others, first make sure that the shoulder straps are in the correct slots, that the harness is very snug, and that the retainer clip is in place at armpit level.
Next, plan to spend one or two intense weeks working on the problem. Bring the safety seat into your home and let your child play "mom" or "dad," and carefully buckle in a favorite doll or animal. This dramatic play begins the process of identification with the best way to behave. Schedule each trip so that you have enough time to pull over and stop the car every single time your child gets out of the safety seat. Each time, explain that you cannot drive until everyone is buckled up. If you act bored instead of angry, she will soon get tired of misbehaving.
Plan some rewards, too. For instance, arrange a special trip to a place the child likes to visit and explain that the car will get there faster if everyone stays buckled up. Try playing audio cassettes your child likes, and rotate them to prevent boredom. Also, give your child soft toys and books to enjoy in the car, but nothing hard or sharp that could hurt someone in a crash. Children look forward to having special toys for trips. Rotating a few items from week to week will help keep their interest.
If your child belongs to a day care or activity group, discuss the need for a buckle-up program for all of the children and parents. There are probably other parents who would welcome this kind of help as well. The excellent "Bucklebear" curriculum and associated materials can be helpful in such efforts."