The incident occurred in the afternoon near Cheung Sha Wan Plaza, involving a route 86 bus that was heading towards Mei Foo.
According to KMB, after the tire came loose, it struck the iron railings near a bus stop.
The young boy, who was seated behind those railings at the time, sustained minor foot injuries and was taken to Caritas Medical Centre for treatment. KMB said that its staff visited the injured child.
Company records show that the bus captain carried out a routine inspection before the journey and reported no mechanical abnormalities at the time.
Stressing that it takes the matter extremely seriously, KMB has pledged a thorough investigation into the cause.
The company will also work alongside the bus manufacturer to conduct further research.
In the meantime, all buses equipped with the same wheel design have been inspected.
They will only be cleared for service once their safety is fully confirmed.