You're a good person trying to do the right thing: trying to return the phone to the right person.
If this phone was stolen, I suggest you call the police on the guy that sold the phone to you as he/ she cannot just sell stolen property.
In the meantime, you shouldn't have said that you were going to talk to the kids mom. The kid will think that he/ she will get in trouble.
Since you already called the kid, you know the number. Do a reverse phone number check to find out the location. I suggest you try: http://www.spokeo.com or http://www.yellowpages.com .
Appear in person and clear up what's going on.
The other thing you can do is call the number and leave a message. Just say you want to see the owner get their property back.
well did u meet the person somewhere, if so ur fuked i guess.
well i always buy from craigslist i always meet them at their house so if there is any problems i know where the person lives.
but i never go alone.