number1nana,
First, let me welcome you to the forum!
You raise a very important question! It would obviously be different had you known the child, even a little bit. If that were the case, you could have made a phone call to report the suspected neglect.
However, when the child is someone that you don't know, there is not much you can do. That leaves the person, such as yourself, with a great deal of frustration and worry, but unfortunately, without a name or even a valid address, it can not be reported to the professionals.
Did you happen to notice if the child knew anyone else in the surrounding area? For instance, if it was in a grocery store, perhaps the clerk knows who the child is. That's a long shot, but it may be something to go on.
If you are a praying person, than that would definitely be something that you could do. You may not know who the child is, but I believe that God certainly does.
Thank you again for stopping by the forum.