Fresno Police say the two were crossing at Alamos Avenue with another child at around 3:30 p.m.
The man and his 10-year-old stepson were hit as a white van turned left at the intersection.
"At this point, we have no indication that alcohol is involved. But we'll be examining the scene and looking for additional witnesses and trying to reconstruct exactly what happened," said Fresno Police Capt. Burke Farrah.
Police say the child is in surgery with critical injuries. The man suffered a shoulder injury.
Investigators say the driver is being cooperative. Officers are checking with witnesses as well as reviewing nearby surveillance footage.
Officials say to expect the roadway to be closed for some time.