It's not discrimination unless you make it about something other than the childs genuine safety, which is exactly what it sounds like. I absolutely agree that the police or cps must be contacted and if they are not, something will happen to that child. If you contact the authorities, you've done all you can do aside from pulling her out of the pool or the street (which you should do, too). If you do nothing, when something does happen, it will haunt you forever. I cannot imagine something like that on my conscience. Anonymous or not, you should act asap.

Why are you even asking this question?
Make numerous anonymous calls to police and CPS each time she is out on her own. Are you going to feel guilty if she gets kidnapped,killed,hit by a car?
What happens if she succeeds in climbing the pool fence, drowns?
You will have personal liability if not criminal because you knew she was placing herself in danger.
The apartment company will face the worst publicity and the lawsuit will last years and a child will have died.
As a manager you are to protect your owners, property and residents from harm.

I agree you should call the police. This would give you a 3rd party verification every time the child is wandering around unattended.

When we had this happen at our property, I had to be extremely careful on wording the notice because we are government funded and can absolutely not state something along the lines of "children must be supervised" because it is age discrimination. I started by looking up our city and state code to see if there was something there that said a person under the age of ... has to be accompanied by an adult. My state unfortunately does not have a code like this. I ended up calling the police a couple of times, and in our lease we have a section that stated no loitering in the halls or the parking lot, I served her a notice because her child was playing in the parking lot and the dumpster area and almost was hit by a car. After the police were involved a couple of times she ended up moving out.

Yes I agree, 911
That child could get hurt or worse

If you don't call CPS and something happens to the child, you will have this on your conscience. I know this is a difficult situation and I don't take it lightly making this call but the mothers actions says it all. Prayers to give you strength to do what is right in your heart

I would take her to the office is she’s outside by herself and call the police.