I'm curious why the 8-bit machines themselves aren't an option here; I'd recommend the Commodore 128, as it was the newest of the Commodore machines and had their most advanced BASIC (so you can actually do graphics stuff without directly manipulating memory) Finding a working one might be a bit more pricy these days than a Raspberry Pi, though.

Otherwise, I'd say the web is probably the closest thing; once you help them over the initial hurdles of setting up a basic site with a simple JS program that they can mess with, it's possible to do quite a lot, in an environment most kids are going to be familiar with; configure a simple Linux environment to open a web browser and a text editor on boot and there you go.