Depending on what you want to measure the electrodes have to be attached to different locations on the head. There is a standardized placemenet system called the 10-20 System.
10-20 System for Kids
http://www.brainmaster.com/generalinfo/generalinfo.html
http://www.piercecollege.com/faculty/pandeyc/ElectrodePlacement.html
http://members.aol.com/aduial/1020sys.html