Neither the girl or the infant were identified in media reports.
Also Monday, authorities were prepared to release the identities of those killed in the crash late Saturday morning along County Road P near the Hwy.
The 11 charges filed against Colten Treu on Tuesday include four counts of vehicular homicide and four counts of a hit-and-run resulting in death.
The mother of one of the girls killed said she was standing near her daughter when the truck came crashing toward them.
Lake Hallie Police said Mr Treu and a passenger in the pickup truck both told investigators they had intentionally been inhaling chemical vapours just prior to the crash. They were picking up litter off a western Wisconsin highway Saturday morning when they were hit by the truck.
A sheriff's deputy investigating the accident traced a fluid trail that led to the house where Treu lived with the truck's passenger, according to the complaint.
"This is a terrible reminder of our nations epidemic of self-indulging with substance abuse without regard of the consequences", Lake Hallie Police Chief Cal D. Smokowicz said in a statement Monday. Her condition has been upgraded from critical to stable.
In court on Monday, Chippewa County Sheriff James Kowalczyk said the girl told authorities that she was holding the child and dropped him, according to the Minneapolis Star Tribune.
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.
The local Girl Scout group consisted of 12 persons (5 adults and 7 children) who participating in a highway cleanup project.
The Chippewa Falls Area Unified School District says it's received a lot of support from the community and even from around the country.
The 21-year-old driver, Colten Treu of Chippewa Falls, later surrendered.