Unfortunately, material you might find objectionable is quite easy to access on the internet.  A simple typo when entering a web page address might land you on an adult-oriented web page.

Role playing is a good way to teach kids how to react in potentially harmful situations.  Ask you children the following questions and listen to their responses.

what if - You were talking online with your friend, and you got a message from a kid who says, "hey, what's your name and age? I think I know you.  It would be really cool to get together at your house or mine, but I forgot your address."  What would you do?

what if - You got an e-mail or instant message that said "I'm new at your school and my parents said I could invite a bunch of kids over for pizza and video games tomorrow after school.  They said we would all meet in the park down the street about 4:00.  What would you do?

what if - You were talking online with a friend that you DO know and some other kid keeps sending you messages saying 'hi' or 'can we talk'.  What would you do?

what if - You were talking online with a friend and he sends you a web link to a site you that is inappropriate saying "check this out!"  What would you do?

what if - You ever DID email somebody you don't know or said things online you think you shouldn't have-- personal information, etc.  What would you do?

It's helpful to remember that the Internet allows people to "pretend" they are somebody they are not.  The "14 yr old boy" they are talking to online may easily be a grown man just saying he is a 14 yr old in order to make contact with kids online.  You have no way of discerning somebody's real identity over the Internet so it is VERY important to ONLY communicate with people you are absolutely sure of who it is.