Kids and Computers

Zac and Isabel both love to play games on the computer.  They have gotten pretty good and can navigate all over Noggin.com and playhousedisney.com and such without any help. Yesterday the two of them were taking turns playing and they had left Safari (which is the operating system on the Mac) open on the desk top when they were done.  I came over and closed out Safari and noticed another screen open even after that.  I thought David and Andrea were playing a prank on us when I read it!  It said "Click to install Porn4Mac"!  Come to find out, David and Andrea had nothing to do with this.  The scary question is, what were Zac and Isabel up to???