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The Internet can be a very dangerous place and kids are very impressionable and can be made/asked to do things without realizing what they are doing..
Teaching them to do the right thing is definitely tough. I have a daughter and her Mac is fairly locked down to a few things she likes to do. She's very young now and hasn't ventured beyond Nick Jr. yet, but that day will come and I sympathize with your situation..knowing that I'll have to go through the same conversation with my daughter at some point in the near future..
Please take a look at this blog post with 10 options for you to try:
10 Parental Control Apps For Mac. One of them is even a free application, so that's something worth trying..
Another thing that a lot of parents do to prevent such situations is to put the computer (even if it's your childs) in a public place like the living room or some other frequently traveled place. The constant presence of another person in the vicinity will make the kids think about what they are doing..
The other thing a few of my friends with older kids have done is going the shock route by showing them the consequences of what happens when the kids start talking to that "15 year cute boy/girl" on the Internet and the next thing they're on a plane to some remote place to meet that person..depending on the age of your daughter, that might be appropriate or not..
Good luck..