Zerynth and Micropython are two completely separate projects. Zerynth has been developed from scratch thanks to the support of a Kickstarter campaign. For a more detailed comparison take a look here: Micropython vs Zerynth
Zerynth takes about 10-15 seconds to upload, and you haven't to to use any script. Take a look here: Python for ESP8288 using Zerynth
For those who are interested in Python for embedded, take a look here: The Rise of Python for Embedded Systems
I hope it will be useful.
MicroPython is a very good implementation of Python on microcontrollers. Zerynth is Python even more tuned to micros than MicroPython is. A Micro MicroPython :)
Take a look here https://www.zerynth.com/faq/ and here: Micropython vs Zerynth for more technical details.
>Pico is maybe aimed at slightly older kids?
Do-it-yourself, make your own educational use. Micro:bit and partners provide a more complete solution (well it started by sending every British kid one for free so...)
>I hadn't heard of Zerynth, do they make their own dev boards? At a glance they seem very industry-focussed.
They support existing boards/microcontrollers like Atmega, STM, ESP - https://www.zerynth.com/integrations/ - and they work by having an IDE with an internal Python implementation - you write code in Python that then is compiled and uploaded to the microcontroller/board as an optimized binary - Python is not run directly on the board. And each dev board that is supported has a nice integration in the IDE (kind of user friendly). And the code is kind of more cross-chip compatible than MicroPython.
And they business model is that you can actually design, launch and manage products based on their stack.