The Facebook Messenger app adds a padlock icon under the profile picture of the sender and next to the message when a chat is encrypted. However, in order to see this, you’ll have to log into their Messenger account from their device.
If you don’t have your child’s Messenger credentials, use a parental control app like mSpy to see if your child is using secret conversations on Facebook. The app gives you access to your child’s messaging activities, even within encrypted chats.