Thanks for the suggestions so far!
This looks quite interesting as a potential base to work from : Sketchbook v0.4 (three.js + cannon.js playground) https://jblaha.art/sketchbook/latest - if could be combined with a tool to build simple tutorials/scenarios and expose tutorial specific properties and methods it could work such as character.walkForward(), car.drive(20), character.onCollision() etc. It would also need a framework to interactively code (like blockly, js or python).
The problem I’ve found with existing sites is that they seem quite limited or they’re often more focused on building a business than they are on being useful.