"Would you share your toothbrush with a friend?" asks Stephen Carrick-Davies, the charity's chief executive. "And what about that photo you've posted on MySpace? Would you want a future employer to see that?"
With government funding, Childnet has made its new "Know It All for Parents" CD-Rom available free through schools - and received 50,000 orders on the first day.
The friendly, clear presentation shows parents and teachers how children are using modern communication technology and how they can help them to use it more safely. The CD-Rom can be viewed in nine languages, including sign language.
Parmjit Dhanda, the children's minister, said: "We should never take it for granted that our childen know all about computers and the internet."