Boy? girl? What do they like doing? Playing games? Graphics?
I'm a bit prejudiced here as I have always used a Mac and my kids and I have Mac Ibooks and Mac Mini desk tops.
The advantage is that they don't get viruses and they don't crash and they rarely go wrong - which is big consideration if you are not technically minded enough to fix problems. They are also unbeatable for graphics (software built in), photo management and music. They are also very 'cool'with kids.
That said, there is nothing like as much games software for Macs compared with PCs.
All routers will be compatible - if you have a big house just go for the one which has the longest range you can afford. There is not a lot to choose between them in quality.
You should be fine. I would expect it to connect on 802.11g, b would be quite slow (max 1mbps IIRC). I forget quite what the max speed is for 802.11g, but it's faster than most peoples ADSL links. That card gives you the option if upgrading the router to n if you have any issues with range or a superfast cable link.