J.
My daughter did this when she was two. We asked a police officer to talk to her about being safe in the carseat and that it needs to stay buckled at all times. He spoke to her through her opened window in the car, and she had her face covered because she was embarrassed, but it worked. We reminded her occassionally that if she took off her seatbelt, we would need to find another police officer to talk to. She has not opened her buckle since. (two years later!)