Child Seat Booster problems of the 1997 Dodge Grand Caravan

The harnesses on the integrated child restraint seats do not restrain my child at all. She can simply wiggle her shoulders through and climb out. She has wiggled her shoulders and upper body through several times and been stuck at the hips. Twice I have been driving over 60 miles per hour on the florida turnpike and discovered that she had completely freed herself from the restraint and was walking around in the van! the harness was still clipped closed, but loose. Obviously, this is totally unacceptable. I need help fast. My daughter is only 17 months old and I will have another baby in June.