The incident happened Saturday, in the 4400 block of Rosemary Lane near Keith Road in north Harris County.
Initial reports stated the two were mother and daughter, but a lieutenant with the Harris County Sheriff's Office later said the two are not related.
"At first, we thought it was a mother and daughter, but we learned after they were transported that it was a 17-year-old and a 6-year-old and they're not related at all," he said.
The 17-year-old died at the hospital and the 6-year-old victim was in critical condition. But on Monday, Gonzalez shared the tragic update that the 6-year-old was determined to have irreversible loss of brain function.
A GoFundMe was created to help the family of the 6-year-old victim cover funeral costs and help her parents.
The driver of the vehicle was also sent to the hospital with non-life-threatening injuries, authorities said.
It's unclear what led to the crash, but photos posted by authorities on Twitter showed a black SUV in a ditch and a mangled ATV.
The video above is from a previous story.