If the president's daughter was kidnapped, they would have sent more than just three police cars. At the very least, the FBI and secret service would have sent several vehicles and blocked off exits.
The two children are tied to each other, but not the chairs they are sitting on. They could have simply stood up and had enough slack to get free.
During the car chase near the end, as the camera pans back to look at the police car there is a distinct shadow of the car and camera in the bottom left hand corner
The police think that they have lost the van in the shadows, even though they had it closely surrounded and there were outside witnesses.