The internet has been filtered in the house for years (their Mum is an IT tech in a primary school) - both via the broadband supplier's filtering and also using OpenDNS. It's not a perfect solution (and obviously can be bypassed as soon as they have smartphones with Internet access), but it's another element to think about... plus, of course, educating them in the right way so that they see it as a good thing to be kept safe, rather than a bad thing that they're being kept away from something they covet.