According to Kowalczyk, the girl said she was holding the baby when she dropped the infant, who hit his head on a footstool. The 21-year-old driver, Colten Treu of Chippewa Falls, sped off but later surrendered.
Lake Hallie police Sgt. Daniel Sokup says the pickup crossed a lane Saturday morning and veered into a roadside ditch, striking the victims.
Chippewa County Sheriff Jim Kowalczyk said the incident was reported at 4:34 p.m. October 30 of a 6-month-old who was bleeding from his head and unresponsive.
The accused was accompanied by her biological parents in court, where Judge James Isaacson ordered her to be detained on a $50,000 (£38,280) cash bond.
In a separate interview with The Associated Press, she said "Haylee would've never made it if she lost her mother".
"He doesn't understand", Schneider said.
The other Girl Scouts killed in the crash were Jayna Kelley, 9, and Autumn Helgeson, 10, officials said. A passenger in Treu's truck told police he grabbed the steering wheel when he saw that Treu "looked out of it".
During her bond hearing, the girl in a "colorful striped skirt, a black hoodie and pink sneakers" was sobbing as the charges were described to her, the Star Tribune reported. He is set to return to court Tuesday afternoon.
Haylee's father died when she was 3, Judy Schneider told the AP, and her daughter, granddaughter and grandson moved in with her several years ago.
She said the support has been positively overwhelming. Now, she said, "It's going to be hollow".
Police say 21-year-old Colten Treu (TROO) had been inhaling chemical vapors before crashing his truck into the group as they picked up trash along a rural Wisconsin highway in Lake Hallie.
This traffic crash resulted in the death of 4 members of a Girl Scout Troop. The girls were wearing highly visible green safety vests while participating in a highway cleanup project.
Hundreds of community members huddled under umbrellas in the rain Sunday evening for a candlelight vigil outside Halmstad Elementary.
Another girl who was struck is still hospitalized but is now in stable condition.
Schneider said her daughter's "favorite time with her kids was all in their jammies, curled up to watch a movie".