A second child was also hurt amid unconfirmed reports that an Uber was involved in a crash with an RACV roadside assistance van.
The Australian network 9News reports that five people were injured in the crash on the corner of William Street and La Trobe Street, after one of the vehicles lost control at roughly 3.45pm local time (6.45am BST).
The exact cause of the crash is unclear, but it is reportedly being treated as accidental.
9News claims that the RACV driver is expected to be charged.
A man in his 30s is in a serious condition with leg injuries, ABC News reports.
9News adds that two women, also in their 30s, are in a stable condition with leg, back and chest injuries.
A pre-school age girl is reportedly a stable condition with grazes, while another child is believed to have suffered minor injuries.
9News reports the fifth person injured was also an adult.