Presumably the content isn't encrypted at rest on the device, or if it is, it could be decrypted by the phone owner (parent, child) to help aid an investigation

Presumably the content isn't encrypted at rest on the device, or if it is, it could be decrypted by the phone owner (parent, child) to help aid an investigation. E2EE is not the issue when one party discloses the info, excepting mechanisms like plausible deniability built into apps like Signal, but even at that point LE should be able to get a warrant and then do proper police work: stalk the target under cover until they unlock their device, then rush in and grab it before they can put it back to rest (e.g, how it was portrayed in the Silk Road movie).