Depending on how computer-aware they are, children of that age can be MAJOR MEGA EXTREMELY curious... Believe me when I say that even a 5 year old can screw up your entire PC (grrrr at my little cousin).
Make a backup of your HDD and store it somewhere OFFLINE. eg: put it on another HDD and physically disconnect that HDD. (and indeed, remove all your stuff and/or stuff they don't need from the HDD which they are going to use - In essence, make a new Windows System for them, like rav0 said).
Also, put a password on your BIOS for sure! (and make sure the bios does not lock you out for ever when you've entered 3 wrong passwords).
Then complain to your parents that a PC is NOT a toy to mess with. It is a machine which needs fine tuning and maintenance like any other machine. I bet they don't let those children play with dad's car or mom's washing machine either...
Personally I would even go as far as building a small PC for them with parts I have laying around, or even buy a very cheap PC they can play with...
The younger they are, the more dangerous they are (think about when you were that age
)
PS: I do love children though
/ 
