The man, who would not give his name, told how the family were on their way to an uncle's wedding where the girl was a bridesmaid and all were dressed up with even the baby boy wearing a bow-tie.
He said: "They were going to an uncle's wedding and the girl was a bridesmaid.
"She stopped to get some crisps and the 12 year old was in the car with the baby.
"While she was in the shop the man jumped into the car and drove away.
"The girl climbed into the front of the car, it had three doors, and threw herself out when the car was moving.
"It was amazing how the 12-year-old managed to open the door. She was wearing heels as well.
"She just rolled out as if she was in a film and started screaming.
"She was in floods of tears and was screaming, 'the baby'.
"If she hadn't got out, the mum would have come and out and seen the car and both of them gone and would not have known what happened.
"She has a scratched arm and bruises because the car was moving, but was otherwise physically unharmed."
The man explained how both the mother and stepdaughter had iPhones in the car and the police and a shopkeeper tried to drive around following the phone signal.
Police used a helicopter in the search for the car which was found nearly six miles away in Cheshunt without the baby inside.
After an agonising hour, the family were told the baby had been found abandoned on a doorstep in Cheshunt.
He said: "The baby had a bow tie on, all dressed up for the wedding when he was found. It was about an hour later.
"A lady came out and found the him on her doorstep. She brought him in, she changed his nappy and called the police.
"The mum is absolutely traumatised. She's in real distress."
Shopkeeper Sultan Salman, 42, from Enfield Food and Wine, said: "A woman came into the shop.
"She was wearing a dress for a wedding and I asked her if she was going to a wedding and she said yes.
"She was getting two orange drinks and some polos.
"Then we heard someone screaming outside and I saw a woman in a similar dress, screaming: 'the baby, the baby'.
"A little girl in a green dress, they were both in the same dress, was sitting down and shaking for about ten minutes, not saying anything.
"The woman was asking, 'did anyone see anything' and I said I didn't.
"About half an hour later, a police car came to help.
"It was a big big thing to have happened.
"It was horrible."
A couple have spoken of their shock after finding a little baby in full formal dress dumped on doorstep of their flats by a carjacker.
The baby boy, who turns one today was in his mother's car with his 12-year-old half-sister outside a shop when a thief jumped in and drove off.
The girl, due to be a bridesmaid at a wedding later in the day, jumped out of the moving car but had to leave her brother behind.
The infant was left on the doorstep of a block of flats six miles away from where the car was stolen in Enfield, north London.
Luckily for the child, a couple, who had a baby of their own, took him in, changed his nappy and phoned the police.
The relieved parents collected the baby in tears and even managed to make the wedding reception.
Christina Koustouda, 32, a food technologist, was on her way out when she spotted the baby sitting up leaning against the front doors of her block of flats.
She said: "He was screaming and crying but he was leaning on the door so I couldn't open it without him falling over and banging his head.
"The door opens outwards so I had to get down on my knees and open the door slightly and hold the baby and then gradually push the door open.
"It was really weird, there was no one around and he was dressed very smartly like a little groom with a black suit and bow tie.
"My husband called the police and they were there within two minutes.
"We went up into my flat and he was still crying like crazy.
"I have a 14-month-old daughter so I changed him into one of her nappies and gave him some water and tried to calm him down.
'The mum and dad arrived after about half an hour later and it was only then that he stopped crying, but then everyone else started crying.
"The parents were so relieved to see him, the mum was screaming and crying, the dad was crying, I was crying, my husband was crying and even the policeman started crying. It was like a scene out of a film.
"The mum was so grateful she kept thanking me over and over.
"It's the little boy's birthday today and we've been invited."
A spokeswoman for the Met Police said officers were still trying to trace the suspect.
She said: "No one has been arrested yet. We are doing the usual house-to-house enquiries, so hopefully we will have an update soon when we have a fuller picture.
"But at this stage we don't know the motivation for the incident as we are still trying to trace this individual.'
