The assaults continued until the latter part of 2016, Riverside Deputy District Attorney Sean Oswill said.
Welch lived with the victim's mother, who also had two young sons, the Press-Enterprise reported. According to court records, Welch began groping the girl when her mother was away from their apartment or asleep.
The co-worker's concerns led to an investigation and interview with a county forensic examiner, the Press-Examiner reported. A follow-up medical examination was curtailed when the victim's mother interfered with the process, the newspaper reported.
Welch was arrested March 17, 2017, KTLA reported.
Schwartz, during the sentencing, called the case "a complete travesty," the Sun reported.
"There was a failing by our social services in this case that resulted in additional harm, both emotional and physical, to the victim," Schwartz said. "It appears the people responsible for taking care of her, her mother and Department of Public Socials Services, failed her."
Riverside County officials agreed to pay the victim a $10 million settlement to drop a civil lawsuit against the county, the Sun reported.