Sorry for not getting back sooner. Wireless-N is about 15 times faster (more like 12) than Wireless-G. If you plan on bandwidth-hungry apps like high-definition video and voice over IP, then that kind of throughput would better handle it. Since you already have the Linksys Wireless-G, I would use it and upgrade only if absolutely necessary.
Since the boys have a card installed, they should be good to go once network is set up. As to the Compact Wireless-G adapter. If you do not have a wireless card installed in your PC, then this is the easy way to connect wirelessly. It is limited to data rates up to 54Mbps (5 times as fast as 802.11b). USB is slow, but an easy install if you don't feel comfortable removing your desktop's cover to install a card.
The Linksys Wireless-G should be ok for Win XP since you had no issues installing it. Don't know what model you have, but most show you in the install manual how to set up security. I would get a notebook and write down what you see on each step before making changes. Just do not use the default passwords. I would use WPA (Wi-Fi Protected Access) security.
Since the laptops already have wireless, you should not need to install anything from the CD. They ned to use their own wireless setup to detect the network in your home and connect to it with the right password, etc..