I google walked her route to the park. From my best judgement, if mom stood at the end of her driveway and watched Jessica until she was out of sight, she would have been able to see her until she reached the yard with the high fence and big swing set. It is about a block and a half down the street. There is where the road curves and so Jessica would have been no longer visible. Also, distance wise, I was able to see a truck parked at that section of the road from the front of the family home (in google world it is a white suburban-looking vehicle, just for reference). At that point, Jessica only needed to walk an additional half block to reach the park. She should have been visible to anyone at the park from that vantage point. There have to be witnesses. It is such a small section of road and very heavily settled.
Just thinking about how quickly a child can disappear... It's not fair...