See Help and Support and search for "take ownership"

HOW TO: Take Ownership of a File or Folder in Windows XP
http://support.microsoft.com/?id=308421

You cannot password protect individual folders in XP. You restrict access by
assigning permissions to drives, folders and files.

To do this, you must be running NTFS as your file system on the drive in
question.  If your hard disk/partition is not NTFS you will need to convert
it.  To do this, open a Command Prompt window and enter the following
command:

CONVERT X: /FS:NTFS

Where X: is the drive letter you wish to convert.  You may also want to see
http://www.aumha.org/a/ntfscvt.htm to ensure that you're getting the optimal
conversion.

After this step is completed and you've rebooted the computer, if necessary:

XP PRO:  In Windows Explorer, go to Tools, Folder Options, View and uncheck
Use Simple File Sharing.  Now, when you right click on a drive, folder or
file (on an NTFS partition) and select Properties, you'll see a Security
tab.  Here you can assign or deny permissions based on user name or user
group membership.

XP Home: By default, you can only make files and folders under My Documents
"private". This is done by right clicking a folder or file and selecting
Properties, Sharing.  To change the permissions on other folders, you need
to boot the computer to Safe Mode and log in on the built in Administrator
account.  In this mode, you'll see the Security tab in Properties, and you
can assign permissions based on user name or group membership.

How Do I Get the Security Tab in Properties - XP Home (makes the Security
tab appear outside of Safe Mode)
http://www.dougknox.com, Win XP Tips section

HOW TO: Set, View, Change, or Remove File and Folder Permissions in Windows
XP
http://support.microsoft.com/default.aspx?scid=kb;en-us;308418

HOW TO: Set, View, Change, or Remove Special Permissions for Files and
Folders
http://support.microsoft.com/default.aspx?scid=kb;EN-US;308419

HOW TO: Disable Simplified Sharing and Password-Protect a Shared Folder in
Windows XP
http://support.microsoft.com/default.aspx?scid=kb;EN-US;307874

Your child has likely made the folder private.
This means their account has become Owner and the
only account granted premissions on it and the contents.
To change this you either need to
1. log in with your child's account and issue at cmd prompt
cacls ":\documents and  settings\<child's name>" /t /e /g Adminsitrators:f
or
2. logged in as an admin take ownership over the folder
following links Doug has provided.  While doing this you
will be prompted for having the premissions reset for you.
Here you should say No, and then when it has completed
issue the cacls command given above.
If you already have taken ownership, and you said yes, then
your childs account is now inoperative as it will have no
premissions on its profile, so you need to issue at a cmd prompt
cacls ":\documents and  settings\<child's name>" /t /e /g system:f
cacls ":\documents and  settings\<child's name>" /t /e /g <child's name>:f

Note: replace all instances of <child's name> above with the
real value of the folder and account login value.