I think this is an amazing idea! I played with Basic and Visual Basic when I was younger and it taught me all about logic loops, basic data structures, etc. Putting that type of playground on the web sounds interesting. Have you started yet?
Here is the Duck source code: https://github.com/gregtour/duck-lang/tree/duck2 The documentation reflects the original version, but duck2.cfg shows the new grammar and the /js folder shows the current progress.
I have a mockup of the interface here as well: http://brainplex.net/gamepad/
One big issue is finding time.