Motivation for learning C++ when you know Java
I think you're having trouble justifying the study of C++ because of the way that you're thinking about it.  If you are aiming towards the practical benefits, then you immediately get to the following trap: neither you nor your students has any idea what languages they will work with in the future, so neither you nor your students have any idea what languages would be the most practical to learn.  There are no good answers when you begin this way, and the motivations will always seem to fall a bit flat.

I might steer the students (and you!) towards a slightly different model of thinking about benefits here.  What foundational ideas about programming and Computer Science are easier to grasp in C++ than in Java?  These foundational ideas will important later, regardless of the development environment, because they are the building blocks of computational thinking.

When we teach CS, we are concept-focused and language agnostic.

This lens also provides you with the beginnings of a map through the topic itself.  You can quickly sketch out a curriculum at this point, which is best done backwards:


What are the most important concepts that you want the students to learn? 


Fair warning: I am not a C++ programmer!  There may well be more important topics than these to get into, but I know that C++ gives students an opportunity to learn about pointers, pointer arithmetic, and memory management.  You could also get into casting and bit-operations, which forces the students to learn about data representation.  Each of these concepts are fundamental to computers, and much harder to learn about in Java, which abstracts them away.

Selling the language to the students is now a comparatively easier task.  You're using C++ because you get to show them some very cool things about how computers really work.

The best time to do such selling is on the first day.  By explaining #1 to them at the beginning, you alter their motivation going forward.  They want to do well on the tasks you've outlined not only because it gets them a better grade, but because they are also mastering deep, important concepts that will help them understand computational thinking.  Computational thinking will help them regardless of what they do in the future.  Heck, they could leave the field entirely; they would still have mastered a rich and powerful tool-set for thinking about problems, wherever they go.

Now we get to the planning portion:


What is the most logical ordering of those concepts?
How will you (and the students!) know that the concepts have been learned?  This could be some combination of projects and assessments.


I've grouped these together because they are a little interdependent. Sometimes step 3 will cause you to go back and modify the order in step 2, but these two steps are how you arrive at a well-organized semester.  The best tasks for #3 are also rich and interesting in their own right.  (One resource I could recommend for help finding great, motivating tasks for a given concept is this very site.)

It is extremely important that you really get a good answer for #3.  If you've done these three steps well, then the projects and assessments will now be sufficient to prove that the students understand the material.  If you've truly answered #3, and you have total trust in that, then it frees you up to spend the entire course doing one thing:


Prepare your students to do really well on those tasks.


The reason that #4 is so important is because of the social dynamic that results. You really want your students to feel like you're on their side, pulling for the same team. And if your goal, every day, is to help them master the material, then they will be able to see that you are putting tremendous effort into trying to help them.  And there are also benefits to you here; as a relatively beginner teacher (who will certainly make some mistakes along the way!), having students who are willing to cut you a little slack can make your life a lot easier as well.

Now we have a strong course structure!  Tell the students about the cool concepts they will be learning, and tell them that C++ is a great language to focus in on these concepts.  Map out the order of the concepts that you'll be covering and tell them about the projects, tests, etc.  Then come in every day, and focus in a rather laser-like way on making sure that the students are well prepared for the important tasks that you've designed.

(Apologies for the length of this response.  I wasn't trying to write a book chapter, but I am leaving the information in in the hopes that you will find it very helpful!)