wireless networks are based off of 2 technologies... the AD HOC and the infrastructure....
a quick way to explain this is :
adhoc= wireles nics networking the pcs to each other
infrastructre= nics in pcs and an access point connecting them to the network =nics and a router or hub whatever.
if you want to save cash, get 2 wireless g nics and network the pcs that way; if your not going to network over 10 to 16 pcs.
, you did say the router you are currently using is already attached to a pc (hardwired) for internet purposes. the only thing is the pc that has the router hardwired into it will also become another router (your sons pc will use it to access the hardwired router that you use for the internet) ; so it will have to be powered on at all times.
the other way is... spend the cash and get a wireless g router and 2 g nics. then both pcs can be on and off as they please whilke accessing the internet. your going to plug your internet router into the wireless g router, from there it will shoot toward the pc's.
i did it the first way i specified but, my other pc that stays on all the time is also a server, i need that machnie running 24/7.
i'd like to suggest that you should learn about security while you jump into the wireless networking. someone who knows what they're doing or on some instances, pcs auto configure theselves to pick up on the network = your neighbors pc may attach itself to your network for free internet access.
have fun!