Java or Kotlin for beginners (procedural programming knowledge is existing)?
I've been using Kotlin for two years now on an introductory course on imperative programming (bachelor's first semester) and I'm very happy with this decision. Before Kotlin, I was using Java for the same course.

Here are some of the benefits:


No need to explain what are classes on a imperative programming course, since Kotlin programs can only have a main() top level function (like C)
It is a "safe" language in the sense that it prevents most novice mistakes at compilation time (uninitialised variables, type conversion errors, etc.) 
For simple interactive programs (e.g., ask two numbers and compute their sum), you don't need all the complexity of the Scanner/BufferedReader classes, just use readLine() and println() (without the System.out). This makes the initial learning curve much more pleasant.
It forces the students to think about the little things that are essential when you are taking the first steps in programming: mutable vs immutable variables, nullable vs non nullable types, mandatory variable initialisation, implicit vs explicit types, conditions as expressions, etc...
It is easy to jump to Java in later courses (than say Python or Javascript)


Most of all, from my experience students find it more motivating to learn to program in Kotlin than Java, since they have to write less to accomplish the same things while at the same time feeling protected about runtime errors.

My advice: go for Kotlin.