Monitoring and controlling wireless useage - How do I?

I have a Netcomm NB5Plus4W direct connected to my desktop running XP
home.

The kids have a desktop in the other room with a wireless card and it
accesses the internet and my printer thru the wireless connection to
the modem.

Can anyone point me to a link that'll explain to me how to set up the
kids computer connection or a network such that I can monitor the
data useage on the kids' computer (been throttled twice already due
to excess downloads) and control their access (cut them off after a
download limit or exclude certain sites, P2P, Messenger etc).

At the moment I only have Bandwith Monitor on my desktop telling me
what througput I'm getting on my computer but not theirs.

Any help appreciated.