Make sure the cd drive is the first device in the bios boot order. Then insert the XP CD and reboot. You should now be able to boot to it.
Don't know what the kids did to your computer, but if you can't get it to boot at all (other than to the cd), you can do a repair installation:
Boot to the XP CD and choose the SECOND repair option, allowing XP to install on top of itself. After completion you'll need to go to Windows Update and download & install all updates (except for hardware & driver related updates, which should never be downloaded from Windows Update - only from the OEM websites). Here is a clear tutorial on how to perform a repair install: http://www.geekstogo.com/forum/topic/138-how-to-repair-windows-xp/