BBC News, 01/10/14, parents should be aware that their children can be tracked online, the former head of MI6 has warned.
Sir John Scarlett said children and teenagers were more relaxed about the information they gave online than those even a little older.
He also said the public should be more worried about threats from terrorists and international criminal networks than the government snooping on them.
Sir John was head of the British Secret Intelligence Service for five years.
Speaking before he addressed the Headmasters' and Headmistresses' Conference (HMC) annual conference in Newport, south Wales, Sir John said the technological environment had changed rapidly.
"You've got to know what your children are doing. It's very difficult to know exactly what they're doing in particular when they're on a tablet or something they've got and you've got to have some kind of idea.
"They are extremely vulnerable, everybody is, to a whole range of things. Clearly when they're young children they're particularly vulnerable to predators."
Sir John went on: "Personally what worries me, in a way, most, is tracking devices.
"The way in which locational apps, for example, are now quite freely available, of course you can start off by consciously giving out that information, but once you've done that, you've lost control of it."
There was a need for everyone to be aware that, once information was shared online, for example through using a search engine, it could be used by different firms, Sir John said.
Youngsters could also be tracked through getting into conversations online, or by using apps that used an individual's location.
"It's the tracking, and obviously if you get into conversations, I don't really see how that can be controlled unless you have some idea what your child's doing."
The "basic advice" was to teach children common sense values, such as being aware of the information they gave out and how they expressed themselves, Sir John said.
"If you're a young girl you're obviously vulnerable to tracking," he suggested.
There are individuals who are able to track someone "right down to more or less precisely where you are".
Sir John said that teenagers and younger children were "much more relaxed" about what they shared online because they had grown up with the technology, and it was a normal part of their lives.
Even a "tech-savvy" 30-year-old was less relaxed about what they shared.
He also argued that the public should be less worried about widespread government snooping.
"I think we're worrying in a way about the wrong thing.
"Potentially that capability for mass and uncontrolled snooping is clearly there. Technically it can be done."
The former spy chief warned that in general, there was "no absolute protection" against a determined online attack.
The public could protect themselves by using passwords and other measures against 80% of intrusions, he said, but "everybody has to understand that you can't be absolutely certain that somebody somewhere won't get it".
Sir John, who was chief of the British Secret Intelligence Service until 2009, said: "There are authoritarian states out there, there are terrorists out there, there's a massively growing organised criminal international network out there."