"Do I have a moral obligation to ensure the children's safety?"
When I was a child and terrible things were happening at my mom's house, I used to be terrified and wonder why no one was coming to help. When I grew older and discovered that people did know what was going on but all had their reasons for not helping, I felt like I wasn't worth anything.
Kids need protectors...even against relatives. If the only option is Children's Services then use them.
You have no idea the anguish in a child's mind when that child thinks no one can save him/her....that nobody sees or cares.
You can always call them and say you're not sure, but you were told.....then leave it to them to handle.