I would not call authorities unless there's an absolute good reason for doing so unless you're planning to move.
People retaliate viciously if/when they find out someone called child protection services on them.
Can you mail an anonymous letter to the house of the mother of the child? A typed out letter-no return address-sent through the postal service?
I don't know what you would write though. Maybe an article about child neglect.
Nothing specific about being in the parking lot 'cause then they could deduct who it could be. Keep it very general but to the point of she's neglecting her child.
That's it. See what happens. Mail it as far away from your neighborhood as you can-don't mail it in the same community you live in. They can look at the post stamp and see it was mailed in their neighborhood.
That's the only thing I can think of.
If that fails and things continue or even get worst then send an annonymous type written letter,with no return address, to the police. Again, make it very generalized so the police won't even know you live where you can look over and see the parking lot where these incidents are occuring.
Protect yourself as best you can.
There will be no living there if anybody finds out. I would also fear for your safety if they got a lot of family around.
Take care of yourself.