Monitoring a wireless network?

I doubt Wireshark is the product for you even though it may be an option to capture what your teenagers are doing on the network. Keep in mind that you'll need to buy at least one AirPCAP USB WiFi adapter if you're about to capture WiFi running Wireshark on Windows.

I have no idea what kind of passwords you want to sniff (and why), but I guess you won't be able to see them in at least some if not most cases. Most logins teenagers do today will be web page logins and those are usually SSL encrypted which you can't break unless you are in possession of the private server key (which I doubt you are).

Maybe an alternative option (if you really have to spy on the network traffic) is to install a transparent HTTP proxy that is between your teenagers and the internet. It can be setup to intercept and forward any call to web servers while recording the URL and access time in the proxy logs. Those logs could then be inspected/analyzed by hand or log analyzer software.

Another software you might find usefull is Cain & Abel, but it is to be considered a hacking tool (or, a "security" tool if you want to call it that :-)). It can capture and (often but not always) decrypt passwords on the fly, but it is a little complicated to handle since the GUI is an impressive example of being quite non-intuitive for some activities.

I'll add that you may wish to look at a different router. If you have the inclination I believe that you can do a lot of what you're looking for by using a router running dd-wrt. There is no, real, dummies guide for it and it can be very complicated. I'm running an Asus RT-N16 with dd-wrt and it has great logging capabilities. Jasper's suggestion of Cain&Abel is great, however, as he mentions, setup is complicated. Good Luck!