Shoutbox

The best way to do this is to just navigate to the folder when they are away from the computer...

The default folder is:
C:\Documents and Settings\ *Windows Username* \My Documents\My Chat Logs\

Better yet, use the "Open Chat Logs" option from the messenger plus menu to be able to find certain dates and times (ie only look at dates since you last checked) and the find button lets you search for certain words...

I would say, other than sit behind your kids, this is your best option.