The child was playing in the Iniscarn Crescent area of Derry at about 4.45pm when the device was thrown at him.
It struck him on the back and caused burn injuries, which are being treated in hospital.
Police believe two boys, described as around 12 or 13 years old, were involved in the incident and police are appealing for them or anyone who can identify them to get in touch on 0845 600 8000.
There was no ongoing disorder in the area at the time.
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