You have a lot of good options!
Python mode is wonderful, and I love working with it – it is also a great learner language. But there are some limitations to be aware of – many Python tutorials and online reference materials these days are focused on Python 3, but the built-in Processing Python mode is 2.7, and it is actually running on JVM (Jython) under the hood when it is transpiled – so there can be some gotchas. There are also nowhere near as many tutorials and examples for Python as there are for the default Java mode or for the p5.js JavaScript modes. Still – it is a real pleasure to code in.
p5.js: Sometimes students can be excited about the ability to share their creations – for that, p5.js is ideal, because it can always be published to the web without needing special exporting steps or infrastructure. There are also a lot of p5.js tutorials. In terms of a broad availability of multimedia libraries, options for p5.js aren’t always quite as robust as the Java mode, but that shouldn’t be an issue for your intro.