Police are investigating two incidents in which stones were thrown from the bridge over the M25 near junction 30.
On January 11 a woman reported damage to her windscreen around 4.50pm when something was dropped from the B1335 Stifford Road bridge over the M25 near junction 30.
At about 1.30pm on Saturday afternoon three children were spotted on the bridge throwing what were believed to be stones onto the carriageway of the M25 at the same location.
A lorry driver later reported that a stone hit his cab windscreen causing minor damage.
In both incidents police immediately searched the area, using both local officers and the force helicopter but no one was found.
Chief Insp Justin Smith: "This is very serious and it is fortunate that these incidents did not result in any injuries. These children may think it is just a game but it is so very stupid and dangerous.”
The children were described as two boys and a girl and she was wearing distinctive pink trousers.
They were thought to be two teenagers and a younger child.
Mr Smith added: "I would ask parents to please talk to your children and make sure it is not them that are responsible, because this is a crime and we will arrest anyone found committing these acts.”
Anyone with information is asked to contact police at Grays police station on 101.