It's probably not facebook which is making the problems, it's more likely what they are installing or, already have installed on your PC that is causing the issue.
There are so many levels of blocking on a Windows PC it is a minefield and how far you have to go depends on how technically proficient the children are.
But, I will stress again, they are almost certainly right in thinking it's not facebook making the problems.
If you still want to block it, google for "net nanny" and phone some of those companies up.
The other way would be to make yourself an admin, restrict their user rights, use a firewall, hosts file to block facebook and system policy to stop them changing it back again, but it's very involved and complicated, which can be seen by just how many companies get it wrong and enable you to right click on "cmd.exe" and run it from "open file" dialogs, as one of many examples...
If you want it to stop going wrong, get a knowledgeable geek to reformat it and make you the admin account and the kids to have limited user only accounts. Make sure it is a geek you trust, because if you take it to a PC shop they will 9 times out of 10 root through all your files, copy down your credit card numbers, family photos, etc.
Oh and the admin (that's you!) shouldn't install anything except programs from companies like Microsoft they bought from the shop, not things they download from the internet (it's not strictly true, but if you don't know how to tell the difference, the only [fairly] safe way is to treat it as true).
