Just put a boundary network device with a sniffer, something like a Linux firewall. Allows you to record all IP traffic flowing through it using something like tcpdump. Doesn't need any antenna.
I kept reminding my kids that I could, if I wanted, see the URL of all web pages they visited and a lot of their other traffic over the household LAN (can't do much about 3G, but that's another story). Made them much more 'net aware. I know that they could obscure the data using encryption or a VPN, but this would actually achieve part of what I want them to do, and that is to understand what it is to be on-line safely.
For the record, although a lot of the URL and connection data the data is kept on the Firewall for months, I've never felt it necessary to snoop on them (although I have used the data to prove I could, and also to resolve bandwidth contention between them). It's amazing what being open about what you could do can achieve, without actually doing it.