Just to clarify one possible misconception, the two ends would be the mother and daughter’s phones in this case. Meta shouldn’t have the key to decrypt accessible to them.

You’re absolutely correct that it can be decrypted on either end but Meta should resist putting a backdoor in their app that allows this. If no other reason than it compels them to be in the middle of this criminal case.

Also Facebook Messenger already enables this (https://www.facebook.com/help/messenger-app/1084673321594605) but it’s not the default. It should be.

This is just basic privacy and for sure won’t protect you from a focused government attack but it’s a start.