@Jack Lee while I agree with you in principle I don't think there is any way to effectively implement your idea(s).
You can put a full-screen warning out and it won't stop anyone from doing silly things like opening up the router to the WAN.
@thelonelycoder is correct that the core issue in your example is that the kid was either given, guessed or simply read the password from the bottom of the router. Game over.
I too like to help out family and friends and I give them a printed copy of their router settings and security options to put in their safe box, but I also give them a few copies of the 'guest' networks credentials to be used by not just guests, but their kids and the kids friends too.
The minute I see the teenagers have access to the router? My 'support' has expired. And they already have in writing what that entails with regards to security (with the router settings I print off for them, there is also an overview of what and why their router is 'more' secure with the setup I delivered).
The 'kids' in my example above are younger (up to 15 years old), with a 'public' computer(s) in the living room for homework and no need on their personal gadgets to access the LAN at all). This may not be the same for all families. But I like to educate my family and friends on what they can do (while they can) to protect not just their kids, but also their own digital assets too.