Neat! I think your choice of C and Python as the languages to be taught is very right. The idea of showing the use of C in both Desktop/Server and Embedded environments has long been the approach i had advocated. C is truly the de-facto "universal" language and students should be made aware of it from the start.
I would suggest the following additions;
* The Arduino "language" is C++. Use this as a gentle introduction to C++ as a "better C". From there you can move on to proper C++ (do NOT teach "Modern C++" in the beginning). The intent is to show how "C + some syntactic sugar for expressing abstractions" is quite powerful and that is what is C++. This should prepare the students to embark on a proper study of C++.
* Instead of using the Arduino "language+library" through the IDE, show them how to use the same GNU gcc toolchain to program the MCU directly in C. See Make: AVR programming by Elliot Williams for details. This teaches the students the idea of a "cross compiler toolchain" and all other related matters from first principles.