Automatic Transmission problems of the 2006 Buick Rendezvous

On June 12, 2006 I was leaving my work place with both of my children. I was in the process of strapping my daughter's seat belt on her (age 5) when my 2 year old son jumped up into the driver seat and pulled the vehicle out of park. The car immediately started to roll backwards, as it was parked on a gradual slope. I was trapped behind the left side passenger door until the car made a slight turn and I was able to get out from behind the door. With the door still open the car then made a hard right turn and the passenger door opened where my daughter was sitting caught the tree and bounced back forward approximately 10 feet where it came to rest. I ran down to the car got my son out of the driver seat and pulled the keys straight out of the ignition. The motor of the car was not running at the time either. I phoned the dealership the next morning and they told me that there is no way that the shift lever can be pulled out of the parked position without the brake being depressed, which it was not as my son was standing in the front seat the entire time. They then called gm and I was told to call the customer service number. I called and after 2. 5 weeks of waiting gm is claiming that this particular car, 2006 Buick Rendezvous can be pulled out of park if the key was in the accessory position, which I do not believe it was because the radio would have been playing which it was not. Therefore gm is taking no responsibility for the damages to my vehicle.