OK. I see how you are set up. It doesn't appear that the KPN box has any kind of monitor software or is snmp enabled which would make like easier.
To see who is on line:
Open an elevated command prompt and type:
You should get something like:
Interface: 192.168.1.102 --- 0xb
Internet Address Physical Address Type
192.168.1.1 68-1c-a2-01-af-6a dynamic
192.168.1.101 b0-10-41-4b-fa-f1 dynamic
192.168.1.104 18-cf-5e-5a-b9-3a dynamic
192.168.1.105 e0-cb-1d-43-ab-50 dynamic
192.168.1.255 ff-ff-ff-ff-ff-ff static
126.96.36.199 01-00-5e-00-00-02 static
188.8.131.52 01-00-5e-00-00-fb static
184.108.40.206 01-00-5e-00-00-fc static
220.127.116.11 01-00-5e-7f-ff-fa static
255.255.255.255 ff-ff-ff-ff-ff-ff static
You can ignore the static entries. The top entry (.102) is my PC. .1 is the router. If you have netbios running and all of your house machines are windows then you can use nbtstat -n to see more about them.
You need something which can look at all wireless traffic. Wireshark might work but you would need a wireless adapter that has the right mode and it's not really super user friendly. Gives you too much info. See:
I thought netstumbler might work but it doesn't like my adapter so I can't test it.
Perhaps PRTG in packet sniffing mode might work with wireshark to give you pretty graphs. The freeware version should work for you.
I don't really have time to test these today but I think this is an interesting idea so am going to look into it more.