And locking down the primary device a kid has access to creates the lure of the forbidden; that won't last any longer than their access to another device without those restrictions. That also creates an environment where if they do find something they really ought to be able to talk an adult about, they can't talk to a parent about it because they'll be in trouble (and get their friends in trouble).
One of the widely studied aspects of child psychology is how to instill guidelines that last even when they're elsewhere without any enforcement other than self-enforcement.
> There's nothing in that statement that relates specifically to kids.
Most adults probably don't have parental controls on their phone...