trace network traffic

I have a client who is wanting to see all INbound and OUTbound traffic that going through the network. This is a family that is wanting to do this and they dont want to install software on the computers due to the others are quite computer literate so they want to see if there is a way to do it at a network stand point.

Can't they just monitor it through their firewall?

There modem/router does not support logging. Like I said its a family thing and they want to see what goes in and out from all the computers. If software needs to be installed it can only be on the parents computer.