“Put the laptop down and step away from the pron” usually works in my household.
Seriously though when our 10 year old uses the laptop he’s either in the living room or the dining room, and whilst neither wifey or myself directly observe what he is on the sounds effects are often the giveaway – the click of lego bricks on Lego Digital Designer for example, or the incessant music on Club Penguin. Homework websites are easy enough, he gets bored after 5 minutes and usually shuts the laptop down!!!
I guess what I’m saying is software is good, but equally important is keeping a kid on their toes, if they use a computer in an open access part of the house they’re less likely to access dodgy stuff than if they’re allowed to slope off into their bedrooms for hours on end.