Is there a way to entirely remove the Messages app from an iOS 12 device?
iOS 12 does not seem to allow Messages to be turned off. But this method should prevent messages from working.

Settings > Content &amp; Privacy Restrictions

And set Account Changes to Don't Allow and password protect that setting by turning Content &amp; Privacy Restrictions on at the top of that screen. She will not be able to add an account (EG iCloud/AppleID) that would allow iMessage to get messages. 

Note that a clever and determined child will often find workarounds to anything and could easily find a web-based messaging service available via Safari.