Your paging file should be on a dedicated partition on an entirely different drive to your system files or left on the same drive and in the same partition as your system files in line with a default Windows install.
You get a massive performance loss by splitting the paging file over multiple partitions or drives.
The reason it needs to be on a seperate drive to your system files is that it can be accessed when required independantly of your Windows environment. It's largely a waste of time having the paging file on a seperate partition on the same drive as Windows since the drive cannot spin up and down in response to accessing one or the other. It ends up by looking for the paging file and your system files on two areas of the same physical disk.
To clarify, the same drive is trying to spin up to allow access to both the system files on one partition and your paging file on the other, hence the necessity to give the paging file a dedicated partition on an entirely seperate drive, allowing independent access.
If you only have one hard disk I'd suggest you keep it simple and go for either two or three partitions. The first should contain your Windows istall and all application software, the second could contain your sons Windows install (which can simply be a ghost image of your own, set up as a dual boot, allowing him to put any software he likes into his own mini-system without molesting yours) and the optional third partition could be set up as a data store where you could change the default path of your My Documents folder to use it.
Ask 100 people and you'll get about as many answers but the above, in general, covers the nuts and bolts of your initial outlined plan. Keep it simple or you'll come unstuck.