Seth Ireland was beaten to death in Dec. 2008, four months after the social worker was assigned to the case.
The social worker went through almost three hours of often heated questioning Thursday. Her main contention is that she believed Seth and his half-brother when they told her -- on two occasions -- they were safe at home.
Reports of abuse to Seth Ireland and his half-brother brought CPS to his school two times in the fall of 2008.
Katie Wettlaufer investigated those reports and both times, she decided to leave the boys in their mother's home. The social worker says the entire family convinced her the home was safe, although Seth cast a bad light on his father's home.
"He told me his dad and his dad's wife used drugs in their home, but there was no drug use at his mother's home," Wettlaufer said of her conversation with Seth Ireland.
Seth's father is now suing the county. His attorney says CPS ignored warning signs and left Seth in danger.
Warren Paboojian accused Wettlaufer of failing to follow up on two abuse reports -- from a school principal who had concern of abuse, and from a next-door neighbor, who heard it happening.
Cynthia Potts warned CPS that Lebaron Vaughn was controlling Rena Ireland and her two sons and reported abuse in progress at least twice. But Wettlaufer says she checked on one of those reports and Vaughn again convinced her nothing was wrong.
"The reporting person that reported this to you is telling you that the boyfriend, the same person that's giving you this information, is threatening the children, yelling and screaming at Seth and you're believing what he's telling you on that day, is that your testimony?" Paboojian asked her.
"Yes," Wettlaufer said.
And she said Carla Manning's report didn't seem to merit significant follow-up, because the principal reported the Ireland boys always had an explanation for the injuries -- like this black eye in Seth's school picture.
"Did you have any reason to believe that the reasons the children were giving to the principal were not true?" asked Leslie Dillahunty, who represents Fresno County in the trial.
"No," said Wettlaufer. "I didn't have any reason."
Wettlaufer admitted she never contacted the principal or the neighbor herself. She'll be back on the witness stand on Monday when the trial enters its third week.