If you suspect the tenant might have died or in trouble, your should first check with her emergency contact, and failing that, call your local police department non-emergency number (not 911) for a wellness check.
You can meet the police at the door, and if nobody open the door when knocked, you open the door and let police go inside. They find whatever they find and report to whoever they'd like to report.
Calling child welfare agency on your own is not effective, because you have no direct evidence of the child was abused. They might ask you a few questions and if you don't have direct evidence (you saw something with your own eyes) then they don't have a probable cause, i.e. fourth amendment of "unreasonable search".
