The youngster from Sheffield is believed to have run into the path of the car, a Skoda Octavia, from the driver’s offside, hitting the wing mirror.
The boy was rushed to Sheffield Children’s Hospital with life-threatening head injuries and suspected fractures to his ankle, collar bone and nose.
Malik Ahmed, aged 58, of Fir Vale, told The Star the car was travelling at around 25 to 30mph through green traffic lights on Owler Lane when it hit the boy.
"The child hit the side of the wing mirror, and the wing mirror broke," he said.
"If it had happened a second later he would have hit the side of the door – he could have been knocked under the car.
"I feel shocked that it’s happened like this. I feel sorry for that kid, and I feel sorry for that young driver."
Owler Lane was blocked while the boy was treated, he added.
The driver of the paramedic car, a 28-year-old woman, was not hurt in the collision.
A spokeswoman for Yorkshire Ambulance Service confirmed the boy was struck by the rapid response vehicle at 6.05pm on Saturday.
She said another ambulance was sent to the scene after the accident, which took the boy to hospital.
"We are looking into what happened and co-operating fully with the police. We are also conducting an internal investigation."
"The driver is very distressed by the incident and is being supported by her manager. We wish the young child a speedy recovery."
