If you'd like to delete the comment or threat from your child's site, here's a link on how to do so:

    http://help.xanga.com/deletecomments.htm

Due to security reasons we can not provide more detailed information about a user such as IP, however, if you feel a crime has been committed, we recommended that you print out the threatening posts/comments. If your child knows who the person is in real life, then we recommend that they take those printouts to local law enforcement (either at their school or to the local/state authorities) and show them the printout of the threat. 

Another alternative is Protected Posting, which allows your child to choose who can read their blog and who can't.

    http://help.xanga.com/caniprotectmyposts.htm

If this comment was posted on the Chatterbox, there is little we can do because the chatterbox program is a third party program that a user puts on their site.   However, if the comment was on a Xanga Post, you can click the username to visit their site.  From there on the left side will be information about the user.