Police said the visibility was poor in the morning hours due to a thick blanket of fog and the two vehicles collided head-on near Bhita village, about 15 kms from here.
The children were in the age group of 5 to 8 years.
The school van, which was said to be overloaded, was carrying students of Arjun Public School, a private school, located near Saha, they said.
The driver of the van succumbed to his injuries later, they said, adding the truck driver escaped from the accident spot.
The injured have been admitted to hospitals in Ambala.
Some of the seriously injured children have been rushed to PGI in Chandigarh.
Senior officials of the Ambala district administration have rushed to the accident spot to help in rescue and relief measures.
Seven of those killed had been identified so far. They are Divya, Rahul, Rishab, Raveesh, Pulkit, Harish and Anu.
Chief Minister Bhupinder Singh Hooda has expressed grief over the incident.