The children, aged two and four, were struck by a car in Upper Gornal, Dudley, this morning.
Emergency services were called to Burton Road near Gornal Tyre and Exhaust garage.
It is believed the children were hit by a VW Polo at around 8.30am.
Ambulance service officials said the injured children, a boy and girl, suffered 'multiple serious injuries'.
Army veteran Paul Lewis, who served a total of 22 years across Northern Ireland, Bosnia and Iraq, was one of the first on the scene.
The 47-year-old, who's now director of Forces Recruitment, said: "I was in my house when I heard the crash and I heard screaming.
"That is when I chucked some boots on and ran down the street.
"I am ex-services so I know first aid. The children were in the road and I went to the one that looked more seriously injured.
"The four-year-old looked like she was in pain. I kept holding her hand and I was telling her she was brave.
"One thing I found remarkable was how quick everybody was there, even the helicopter [air ambulance].
"It was disturbing for me because for 22 years I served in Northern Ireland, Bosnia and Iraq – I have seen explosions and all of that.
"I served from the very early 1990s until about eight years ago. But it is young children so it is totally different.
"It just seemed so quick. Mom was there and she was in bits, she was crying her eyes out.
"It is not a good thing to have happen. In four to six weeks, that is the second accident to happen.
"I just want both and the family to be safe and well soon."
Another resident, who wished to remain anonymous, said: "It was very nasty.
"Everybody came out in the road, the paramedics were here in seconds.
" My friend who witnessed it came running over to my house and started banging on the door.
“I was in the zone when I went out."
The children were treated at the scene before being taken to Birmingham Children's Hospital.
The Midlands Air Ambulance was called but the children were taken to hospital by road.
Ambulance service spokesman Jamie Arrowsmith said: "The air ambulance crew, which included a doctor, split between the two ambulances to help continue treatment to both patients en-route to hospital.”
The driver of the car is helping police with their inquiries.
Anyone with information is asked to call police on 101 and ask for Sgt Ade Brown.