iSitter can work with WiFi-Extenders or larger WiFi networks but they must be configured to create one 'big' network for everything to work correctly.
If you configure extenders to create their own little mini networks, then things connected will be able to see the internet but not necessarily other things on the network.
In the diagram above, there is one extender (Airport Express) connected using the correct 'Bridge' mode and there is one connected using 'NAT Mode'. This has the following results:
A - connected to the bridge mode express - can be Baby or Monitor
B - connected to the main router - can be Baby or Monitor
C - connected to the NAT mode express - can only be monitor, it cannot accept connections as a baby from device connected to anything other than the NAT express
Sometimes you don't know to which access point your devices are connecting, and they can even switch mid stream, so its worth making sure everything is configured correctly.
** Note that iSitter Connect is not sensitive to this kind of configuration, as all connections are outgoing to the iSitter connect server.