Police said officers were called to a row of shops near Singlewell Road in Gravesend at 5.45pm on Wednesday over “concerns for the welfare of a boy”.
Officers found the injured victim who was taken to hospital where he tragically died.
Kent Police said the arrested boy was detained “to establish the circumstances of this isolated incident” but has now been released on bail.
Both children involved are believed to be under 12.
No weapons are understood to have been involved.
A woman told Kent Online she saw boys playing near garages and that police had remained at the scene overnight.
Police added they are not looking for anyone else in connection with the incident.
Supt Nick Sparkes of Kent Police said: “Our thoughts are with the family and wider community impacted as a result of Wednesday night’s events.”