Security and freedom are "platonic" ideals. None of those exists in the abstract, as a real world thing, and you can't find one without the other in the wild (in a Disney-like world, maybe, but not in the real world, in the presense of others, that is people that want to deprive you of either/both, and can benefit from doing so).
Trivially speaking, if some thugs can just come and beat you with no police or legal resources available to you, you don't have either pricacy or security. Both are at their mercy.
You could of course defend yourself, but then you're still getting your freedom through security: it's just that in this case you're obligated to cater get that security on your own.
So, we trade some freedom (giving state the ability to enforce laws, have police) in excange for security. And vice versa.
But in any case, my point above was different: that what TFA descrives is not a tradeoff between privacy and security, it is giving up privacy for no real benefit. If anything, losing encryption costs in both privacy AND security.
When people say "privacy and freedom are more important than security" it's in regards to privacy and freedom being curtailed by the government in order for the government to provide security. Don't be absurd.
Privacy is a form of security though (security for your own thoughts and actions in the own home, and in, on, and near your person). At the very least it is a domain that strongly overlaps with security.
And good security is what gives you the safety to be free.
If you want to sacrifice freedom for security, you might end up putting the cart before the horse.
And of course sacrificing privacy for security is at best balancing 2 different kinds of security. You're not necessarily gaining security.
In this case it means that there's no guarantee that children will actually netto be safer if people can scan private communications.
This is giving up their privacy for nothing in return, or worse, negative restrictions on their political freedom and freedom of expression.