The camp's executive director, Richard "Dick" Eastland, who also died in the flood, did not begin evacuating the young campers until more than an hour after the National Weather Service warned of "life-threatening" flash floods, the lawsuit says.
"The 1.14am flash flood warning was the time to begin saving lives," the plaintiffs wrote.
Eastland received the severe flood warning the National Weather Service sent at that time, but he did not begin evacuating the campers asleep in cabins near the river for more than an hour, the Washington Post reported in July.
The families argued the "tragedy was entirely preventable" and wrote they hoped their legal action would help avert future incidents at camps, a cornerstone of summers for young children in the United States.
"We believe that truth and justice are essential to finding peace - not only for our family, but for every family affected," Ryan DeWitt, the father of deceased 9-year-old Molly DeWitt, said. "We trust that through this process, light will be shed on what happened, and our hope is that justice will pave the way for prevention and much-needed safety reform."
Another family filed a separate lawsuit Monday. Douglas Getten, whose 9-year-old daughter Ellen was among the campers who died, filed a negligence lawsuit against the camp and the Eastlands in Travis County Court.
The seven families' lawsuit recounts flooding at Camp Mystic across its nearly century-old history, including at least two floods that happened after the Eastlands began working there and a 1987 flood at a nearby camp where 10 children died. During that deadly flood, Camp Mystic evacuated its own girls, including those in the Bubble Inn, one of the two cabins where its youngest campers live. That cabin and the Twins cabin were in an area of the camp closer to the river, called "the Flats".
Still, Camp Mystic did not make a proper safety plan in the event of flooding and instead instructed counsellors on the Flats to stay in their cabins in the case of a flood unless told otherwise because all the cabins were "constructed on high, safe locations", the lawsuit alleges, characterising the assertion as a falsehood.
Before and on July 4, the plaintiffs allege, Camp Mystic and its leaders turned a blind eye to a number of factors that could have saved lives - the history of flood risk, the flash flood alert and the warnings from its own counsellors.
"It simply ignored the unmistakable safety threat to its campers and counsellors until this tragedy was unavoidable," the families wrote in the lawsuit. "And 27 young girls lost their lives."
In September, Camp Mystic announced that it planned to reopen for the next northern hemisphere summer - a decision the families of the victims condemned. The new lawsuit cited the move, which it said was made without input from the parents of the girls who died, as an example of the camp "putting profits over all else". Among the critics of the decision were CiCi and Will Steward, whose 8-year-old daughter, Cile, is still missing.
"The camp is ready to move on, but these girls and their parents first deserve transparency and justice," the lawsuit says.
Had the camp taken flood safety steps, their five girls - Molly, Anna Margaret Bellows, Lila Bonner, Lainey Landry and Blakely McCrory - would have started third grade this year, the families wrote. The counsellors, Chloe Childress and Katherine Ferruzzo, would have started college at the University of Texas.
Instead, the families wrote, "They all are gone".