you could try a program like
link -->Returnil
anything they do to the computer will be fixed after a reboot,
you could either always leave the program off and then just use "session lock" when the kids want to use the computer or you can always leave returnil turned on and just turn it off when you want to update programs or install new programs,
or you could give them a limited account so they can't install anything and just password protect the admin account.