Javascript is a horrible language, but it is ubiquitous. You’ll want to spend a little time on html and css if you expect them to do more than print output.

You could focus on TypeScript, which will help them avoid some of the worst things, but then you spend more time on tooling and it won’t just run in the browser console.

Python is a reasonably popular language with a good standard library. It has fewer bizarre quirks like adding two lists of ints together to get a string.

I wouldn’t teach C to a general audience.