I was considering Flask too... It introduces a lot of stuff and layers they won't understand at first, but getting their feet wet on web development would be very interesting.
But that requires a bit more knowledge beyond just Python, it is not necessarily bad and they can learn a lot from it but it might be to complex.
"Hello World" in flask is barely more lines than "Hello World" in the console. If anything it's a good on-ramp for webdev.