Earlier quoted context omitted.
Investigate, absolutely. The cops need to do that as well. However, as someone else pointed out "Child services had the family agree to a safety plan" is not investigating. That's taking action and making a threat: "if you don't agree to this we'll take you to court and let a judge sort it out at your expense."
When CPS is investigating abuse it seems completely reasonable for them to implement a safety plan to ensure ongoing abuse doesn't occur during the investigation. Far better than yanking the kids into protective custody.
I have had jobs that required me to be a mandatory reporter several times in my life, and none of this was even close to evidence of abuse I'd have to report.