Ok yeah agreed . But even then it is best to be with your children while they are on one . You never know .
as the IT kid in my family, i had to fight tooth and nail to get my PC in my room - that was in the dial up days, so i had no internet access. the trouble i got into there with naughty material was brought around by my friends on floppy or CD, so its not something i could have been protected from at all - we coulda looked at it at school or at their houses just as easily.
my advice: dont let them have internet access whenever they want, until they're 16 (when they're old enough to bypass what you're doing anyway)
computers of their own is fine, i reccomend it. gives them something to do. Just unplug something critical to their internet access that they cant plug back in without your knowledge (ethernet cable, power pack to router, etc)
dont bother with software on the PC to monitor their usage, just monitor THEM. if they know you check up on them every half hour or so, they arent gunna risk getting caught with their pants down.
very basic threats: tell them if they get caught looking at porn, or other offensive material that the police can track it right back to their PC, and that if you find out they've been looking at that stuff they will lose all internet access and the PC for a month. take phone priviliges away too, for a repeat offence. (key here is to make sure they know the punishment in advance)