So make sure that they don't have privileges on the machine that would allow them to run TOR? There is no way to absolutely guarantee they won't access material that you don't want them to, but you can make it much more difficult than by doing nothing. And, as others have said - this should go hand in hand with good parenting.
That's kind of tricky. It's in the Google Play store thing, for a start. You can run it off a USB stick also.
Looking at her laptop, I'd have to:
1) lock the BIOS down so it could only boot off the HDD
2) give her non admin access so she couldn't install anything (including Virtual Box etc where she could just create a virtual machine running whatever she wanted)
3) disable the USB ports (which would make using her mouse tricky!)
And if she was really determined, she could just get hold of a spare HDD, install her own OS at a friends house, and swap them over.
So yes, you're right - you can't block it against someone determined if they have physical access to the machine and a will to get past it. Which makes me think I might as well go with the "good advice" approach only anyway.
As I (think) I said earlier, she's not old enough to have much of an interest yet anyway, I'm just doing some preparatory research, I think because I'd just read her school's internet policy - but I guess schools have to be even more careful than parents as they have more kids to look after at a time.