An easy thing you could do is to set up a proxy on the network (such as Squid) and use DHCP to force all of the computers on the LAN to use it. It won't be foolproof unless you block any outbound web traffic that isn't coming from the proxy and that will maybe break things, but this is someone's house and not an IT shop so that's not a big deal.
After that, set up all the phones to use wifi and take the hit in battery performance, or else get everyone ipod touches instead of phones with a data plan. You can't get around the fact that he is paying for another data connection per handset from the phone company.
The *best* thing you could do is sit your friend down and advise him that the world is scary and that you can't shield your kids from everything, but you can certainly build a good rapport with them and answer questions about life when they come up.
Your analogy falls apart in recent years though, when you look at the popularity of the iPad and iPhone. Still closed systems, but more "open" options still can't touch them, sales-wise. Probably because these devices aren't just for geeks anymore, and back in the day, a greater % of the PC-owning public was geeks that wanted to tinker with their systems. Now, the vast majority of people buying tablets and smartphones just want it to work - much like when you buy a car; only a small % of people are customizing it with their own after-market parts.
I would submit that the ratio hasn't changed much. There are plenty of people who are willing to follow some online instructions (they have it so easy, these days) to fix issues and plenty of others who love the idea of rooting and putting different OS images onto their devices. Anecdotal evidence suggests that many of these people couldn't puzzle through IRQ conflicts or work a soldering iron if they had to, but something step by step demystifies it to a considerable degree.
The Wintel stuff suffered from the same Apple dominance but eventually came around when some clear front runners emerged, either in the form of strong word of mouth and catalog advertising (Gateway 2k) or in the form of big box electronics retailers pushing a few brands (Best Buy et al). I'd say that there is one other thing that Wintel had going for it which may be a factor going forward: The lack of Steve Jobs at the helm of Apple.
The next few years should be very interesting in the world of tablets.
For a certain subset of students, it is better to teach them how to figure it out on the fly. But the problem here is that now you are running afoul of the history and civics classes you took which state that everybody is made equal - which, at least when it comes to math, is not the case. Some people are like you, others work best by memorizing formulae and running numbers through them without any further analysis.
So this is exactly like the making of Bon Jovi's video for _Bad Medicine_, then?
Never trust an operating system.