"The Cox family will have their day in court," Costello said in denying the state's motion for summary judgment.
The Coxes contend in their lawsuit that state social workers did not do enough to keep the boys safe from their father, Josh Powell, the husband of missing Utah resident Susan Cox Powell.
Josh Powell killed the boys and himself at his Graham-area rental house on Feb. 5, 2012, during what was supposed to be a supervised visit with his sons.
The Coxes sued DSHS in April 2013, alleging that state social workers and their superiors were negligent in their handling of court-ordered supervised visits between Josh Powell and his sons, who were 7 and 5.
Josh Powell was being investigated at the time in the disappearance of his wife from the family home in Utah in December 2009. She remains missing and is presumed dead. The Coxes are Susan Cox Powell's parents.
Assistant attorney general Peter Helmberger argued Friday that state social workers did nothing wrong in their implementation and supervision of visitations between Powell and his sons.
What's more, their role was not to second-guess the judge who ordered the visits or the psychologist who approved them, Helmberger said. They were charged with making the visits happen, and they did so, he argued.
Trying to hold them responsible for Josh Powell's homicidal actions does not comport with the law, Helmberger said.
But Cheryl Snow, one of the attorney's representing the Coxes, argued that the state had a duty to protect the boys from their father, who clearly was dangerous.
"They had an utter failure to implement the policies they were supposed to," Snow told Costello. "They failed to carry out their duty in a reasonable way."
Costello did not rule on the merits of either argument. He simply decided the questions should be put to a jury.
The case is scheduled to go to trial next month.