A gang aged between nine and 15 carried out three hammer attacks in 20 minutes.
West Midlands Police described the unconnected incidents in Coventry as "extremely nasty attacks".
The gang struck between 2010 BST and 2030 BST in Lady Herbert's Gardens and Cox Street on Thursday.
The gang wore hooded tops and baseball caps, and the youth carrying the hammer was believed to be black.
Three teenagers were hit on the back of the head with the hammer, and all needed hospital treatment.
They are currently all being interviewed at their homes by Coventry detectives, who are also studying CCTV footage.