Firstly Apples do get virus', read here:
http://www.telegraph.co.uk/technology/apple/9355995/Apple-drops-virus-immunity-claim-for-Macs.html
Apple are a over-priced brand, you can get more for your money from another manufacturer, buy a decent PC and if you wanna keep it clean and fast download AVG, Spybot - Search and destroy, CCleaner and Defraggler. They're all free, there is paying versions which do have some improvements. Download CCleaner and Defraggler on to your current computer and do all the options, you'll notice the difference straight away I reckon.
Also there's TuneUp Utilities which improves your computer performance, you do have to pay for it but if one of your kids already knows about downloading torrents they'll hook it up for free and probably get the payable versions of the other software for free.
Get one that has Intel Core i5 minimum, but if you want it modern and up to speed for a few years get a Intel Core i7 and get all the stuff I said about and do weekly or fortnightly or just when the thing slows down checks and you'll be sorted.
You will get more for your money and a better computer if you don't go with Apple.
HP and Packard Bell are the best manufacturer, don't get a Sony they suck balls, I've had two of their laptops and both died from a over heating problem.
Also always check reviews online about the computer you're interested in just in case it's a load of ballsack with a major flaw like dies from over heating.