1) I may be reading this wrong, but it seems to me you're actually talking about two concerns here: overuse, and inappropriate use (last sentence).
We don't worry about either one currently, because our DD9 is compliant and self-regulating in both spheres. For example, with inappropriate use, I was down the hall in another room when I overheard a conversation she had with her friends recently when they were surfing Youtube, and came across a suggested video with an inappropriate word in the title. The tone of the discussion was clearly, "OMG, that's inappropriate, we shouldn't click."
As for overuse, we just notice what she's doing via normal parenting, and if she's been spending all day on the machine, we tell her to put it away and go do something else. Mostly, as I said, she self-regulates, and puts it away after an hour or so of use. She does have her own userid that lacks admin privileges, and parental controls are enabled on her userid that shut her down between 10:00pm and 8:00am.
2) As always, the choice of technology comes down to one question: How do you intend to use it?
Our household is a mix of technologies because we have a mix of usages and preferences. DW and DD both like the portability of their laptops, but they occasionally bemoan a laptop's shortcomings, and settle down to use my tricked-out desktop at times, just as I occasionally reach out to grab one of their handy laptops to do something quick, rather than trudge to the computer room and use my machine.
We also have i-devices (phones for the parents, pod for the kiddo) that maximize portability and give us access to the kinds of apps being designed for those. Since the tablet PC exists somewhere between these two spaces and does neither job as well (full function of laptop/desktop versus portability of an iPod/iPhone), we don't have one, and would consider it redundant.
There is an app called Pages, originally developed for the Mac, that works well on iPad, if that's the tablet you're considering. Personally, when I'm working on a document, I strongly prefer having a large monitor and not having the keyboard take up nearly half my viewable space.
