With the method in the article, you must have NAT enabled on ALL routers. Otherwise you don't get the firewall protection, i.e. blocking of unsolicited WAN to LAN traffic that NAT provides.
You can also do this with two routers. Just put your "teen" on the router connected to the Internet and yourself on the downstream router. The NAT on your router will prevent any traffic from his router getting to yours. But you'll still be able to access things on the upstream router by using IP address.
No port forwarding is required unless you wanted to allow connection from the "teen" LAN to yours, which you don't want, or if you have servers running on your LAN.
You can also do this with two routers. Just put your "teen" on the router connected to the Internet and yourself on the downstream router. The NAT on your router will prevent any traffic from his router getting to yours. But you'll still be able to access things on the upstream router by using IP address.
No port forwarding is required unless you wanted to allow connection from the "teen" LAN to yours, which you don't want, or if you have servers running on your LAN.