Create a new admin for your grandson under System Preferences/Accounts. Be sure to check "Allow user to administer this computer." Then log-out and log-in via the new account and return to the Accounts preference pane. Click the lock if it is closed, enter the password when prompted, select the old admin account and click the "-" square directly below "Login Options." You will be presented with three options regarding what should be done with the Home folder of the old admin account. Click the appropriate box and then "ok." You're done!
If you are concerned about the security of data in your Home folder I suggest that you securely delete the files before you do the above. Don't try to delete the entire Home folder or the individual folders... just the sensitive data files inside the folders.
