These comments are full of people like you smugly assuring us that "because Linux" and "a firewall" are enough. Lotsa firewalls out there running on Intel chips, and there have been no shortage of other mainstream firewall appliances found to have vulnerabilities and backdoors over the years, nevermind all the cable modems with built-in wifi and SPI Firewalls.

If you don't know enough to teach your kid basic security practices(all good security, and the EASIEST, MOST CONVENIENT FORMS OF IT - all starts at the physical level - pick up any textbook on the topic), regardless of mitigations you may or may not have implimented, you don't know enough to be having this argument.