Some of the children in the van were adopted or in foster care and live at a group home registered with the state, said Dee Martinez, a spokeswoman for the Colorado Department of Human Services. It wasn't immediately clear whether they were among the dead.
A photo provided by the State Patrol shows the front of the van crumpled into the rear of a large livestock trailer. Kiowa County sheriff's spokesman Chris Sorensen said the van collided with the rear of the truck on U.S. 287 south of Kit Carson, which is about 130 miles southeast of Denver.
The State Patrol said the adult who died was driving the van and the driver of the truck suffered serious injuries. Seven other people who were in the van were taken to area hospitals, the patrol said in a statement.
Two children ages 3 and 13 were taken to The Children's Hospital in suburban Denver. A spokeswoman said she could not release their injuries or conditions.
A school district in Eads, about 15 miles south of Kit Carson, said on its website that the crash involved a family close to many people in the district.
U.S. 287 is a mostly two-lane highway that cuts across the sparsely populated eastern plains of Colorado with few population centers to slow traffic. It is popular with truckers on north-south trips through the state.
No names of those involved in the crash have been released.