The youth, who cannot be named because of his age, told Birmingham Magistrates' Court he did not realise the brothers, aged two and four, were strapped into the rear of the Nissan Note, even after a high-speed multi-vehicle crash.
District Judge Shamim Qureshi was told the boy, who denies kidnap but has admitted motoring offences, was arrested after armed police arrived at the crash scene on Birmingham's busy A38 Bristol Road.
The court heard the children's father was left crying hysterically in the street after his vehicle was stolen as he delivered food to a friend in the Selly Park area on Saturday February 20.
Another defendant, aged 16, also denies kidnap, after being arrested while fleeing from the crash, which left the two-year-old with a cut mouth and a bruised shoulder.
Giving evidence as his mother watched from the public gallery, the 15-year-old told the court he was equipped with bolt cutters and looking for a bicycle to steal when he saw the Nissan with its engine running.
He told the judge: "I first noticed the car when the driver was getting out with a bag in his hand.
"I mistook him for a delivery driver. I jumped into the car and then (the other defendant) has just seen it and followed."
Asked how he was feeling as he drove off, the 15-year-old replied: "A little bit shaky because I have never really done anything on this big of a scale before."
Contradicting the account of the father, that he shouted a warning alerting the youths that his children were inside the Nissan, the boy said: "There was no shouting, no noise to indicate any other person in the vehicle at the time.
"Before the door even closed I managed to get it into first gear and the wheels were spinning."
Claiming his intention was to sell the car to pay off a debt, the youth said he crashed after being pursued by a police van and seeing oncoming police cars.
Describing the moment of impact, the 15-year-old went on: "I 'sandwiched' the car between two other cars and then ran off.
"The very second I have crashed, the airbags went off in my face. It was smoky in the car and I have blacked out for a quick second. My ears were ringing and then I ran off.
"I was still unaware (of the children)."
Insisting that he only became aware that children had been inside the car when he was arrested on suspicion of kidnap, the 15-year-old said: "If I had noticed the kids, I wouldn't have taken the car.
"If I am being brutally honest, the biggest thing I am sorry for here is taking the kids.
"I am not heartless, I have got feelings. I couldn't hurt a kid."
Prosecutor Shawn Williams then asked the youth: "You had one thing on your mind and that was to get away and save your own skin at the expense of the children?"
The youth responded: "No. It doesn't make any sense. I robbed the car off for money to pay a drug-dealer off. The kids had nothing to do with it."
Asked by the judge if he had heard any screaming after the crash injured one of the children, the 15-year-old insisted both children were quiet following the impact.
The youth told the court: "If anything, whenever I have done a crime it has always been to an adult.
"To do it to somebody younger than myself, it just doesn't feel right."
The trial continues.