Explain that they have options which are pretty secure, but different from the exact UI they are used to (Chromebook, Windows RT, iOS). Then there is OSX, which does run MS Office and is sort of secure, but still lets you shoot yourself in the foot if you go to some length. Or they can stick with x86 Windows and hope for the best. Then be honest about your own time commitment in fixing any messes.
Then let them make a choice like adults. Hopefully they modeled all of the above steps during your formative years.