I've intentionally not read the replies so they don't sway my initial impression.
Yes. You are sticking your nose in where it doesn't belong. Questioning your behavior and doing a poll amongst other people is a classic example of the old adage: If you have to ask then obviously what you're considering doing is wrong.
I've read only the first post where (logically) all the facts are up front. You've admitted you aren't the closest of friends. You've admitted that you haven't seen any bruises, behavior changes or other indications that the child is abused. You don't have enough information other than your own opinion that the mother is stupid and the father drinks and you disapprove of both.
It seems you want to accuse your friend of neglecting or not protecting her child based on your own nebulous assumption. But you still want to be her friend if she proves that she is innocent so you don't want her to know you were the one who brought the poop storm down on her. I have to wonder how you'd feel if someone else did that to you and whether you'd want to continue the friendship if/when you found out about that friend's deceit.
Unless and until you have facts, I recommend leaving well enough alone and simply keeping a watchful eye on the situation. If and when you have concrete facts, then you can act. The proper response is to call the police and accept the fallout from your decision. The police will make the determination as to whether CPS gets involved.
If you still want to take the, "I only want to protect the child" path, then pull up your big-girl panties, make the call and accept the fallout. After all, if you protect a child but lose a friend, shouldn't that be worth it?