The rear window of a moving vehicle was smashed after objects were thrown onto a road, police have said.
No-one was injured in the incident which happened on Friday at about 1945 BST on the Cwm bypass, near Ebbw Vale in Blaenau Gwent.
A group of children were seen running from a bridge over the road. Gwent Police said they are searching for those responsible.
Pc Mike Smith said their actions were "extremely dangerous".
"Those engaging in this sort of activity should understand that their actions could cause very serious injury and they could face the consequences," he added.
"Parents and guardians are advised to reiterate this to their children and ensure that they know where they are at all times."
Any witnesses or anyone with information can contact Gwent Police on 101 or the independent charity Crimestoppers anonymously on 0800 555 111.
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