You have hit the nail on the head. It doesn't matter how good your digital security is, if people are clicking on every malware link they can find. So...
I'm afraid I have to agree. Moving to a mac is the quickest way to cut your management time. I moved my mother to Mac a few years ago (when she was 72) and suddenly the support calls stopped. I understand there are some barriers here... price for one, but then go for a mac mini. With 9 & 26 year-olds in the house, I'm guessing there might be some gaming going on. In that case see if one of the systems running Windows can be isolated a bit (either off net, or by removing browsers -- or hiding in the case of Explorer).
Otherwise, look at the security add-ons for firefox. Keep your anti-virus up to date.
Good luck!