If you keep the Scala feature you use minimal, it should not get too complicated.
First step was to learn SBT, which in itself is simple (building projects, dependencies, plugins).
Later we have them do scala exercises (scala-exercises.org) to understand Scala basics, and the philosophy behind Scala, onboard on Framework (Play/Lagom).
Then we move on to Slick, and then Future API, error handling with Try, for yield comprehension, map and flatMap, collections, and of course implicits.
I work at a fashion renting company and our backends are built on top of Scala stack.
Contrary to popular belief, Scala is actually quite a simple language. Most of our engineers came from PHP or Python, or Ruby and all have expressed how type safety gave them great assurances that everything works if it compiles.
On top of that the behaviour of the language is very predictable! We almost never use debugger because we can predict quite successfully what went wrong in case of bugs.
Lagom was such a nice framework, coupled with Akka service discovery makes building Event Sourced application so simple.
I just had a Christmas dinner alone spiraling on how much of a failure i am given my horrible social skill. But this really cheer me up!