One thing you could do would be to enable logging on the Airport Extreme. (Do this by clicking the Airport Options Button in the Airport tab of the utility.) If you've reserved a given IP for your daughter's computer, you can demonstrate to her that you can look up when she was using the network (instant messaging, browsing, itunes, etc) and what IPs she was connecting to. How you leverage this info is left as an exercise for the parent.
You might also want to ask on the macnn forums. Lots of good answers can be found there.