A fourth girl, 10, was injured.
Ms Schneider said the deaths of her daughter and granddaughter were "still not real for any of us". Judy Schneider said her 6-year-old grandson doesn't understand that they're gone.
"She panicked, didn't know what to do and didn't want to get into trouble", Newell said at the girl's bond hearing, KSTP-TV reported.
The Chippewa County Sheriff's Office said he is being held on four counts of homicide by negligent operation of a vehicle, four counts of homicide by intoxicated use of a vehicle, four counts of hit-and-run involving death and one count of hit-and-run involving great bodily harm.
Judy Schneider, the mother of Sara Jo Schneider and Haylee's grandmother, told The Associated Press in an interview Monday that after the crash she wondered, "What was the driver thinking?"
All members of the group - seven children and five adults - "were in the ditch, not on the pavement or gravel shoulder at the time of the crash" and were "wearing green highly visible safety vests", police said. A passenger in Treu's truck told police he grabbed the steering wheel when he saw that Treu "looked out of it".
Treu remains at the county jail on $250,000 bail, reports CBS News correspondent DeMarco Morgan.
Police said that they received a call from a caretaker saying that they had found the boy bleeding from the head and unresponsive.
On Saturday November 3, 2018, at about 11:41am Chippewa Fire District and the Village of Lake Hallie Police responded to CTH-P north of CTH-OO in the Village of Lake Hallie, Chippewa County, Wisconsin for a traffic crash.
The victims were among seven children and five adults who were working in the ditches along the roadway. They were picking up litter off a western Wisconsin highway Saturday morning when they were hit by the truck. He says Treu does not yet have an attorney.
The surviving girl was hospitalised in Rochester in critical condition.
The girls were fourth-graders and members of Troop 3055 in nearby Chippewa Falls, about 90 miles (145 kilometers) east of Minneapolis.
The 21-year-old driver, Colten Treu of Chippewa Falls, later surrendered.