Most schools and early learning seem to focus on things like coding games such as light bot that teach the fundamentals of programming logic, in combination with more simplified languages like Scratch, Swift, or Blocky. Unfortunately, most of those aren't accessible. The good news though is that a close second on that list is Python, which comes standard with Raspberry Pi's and childrens learning kits. The bad news is that many childrens learning guides may focus more on the visual and gaming aspects of python, which may not be entirely accessible either.
There are a number of free books available, but it may be best to avoid some of the more complex ones and keep things simple and engaging, at least at first.
Some free books include:
[Snake Wrangling For Kids] - written for children 8 or older
[Dive Into Python]
[The Python Practice Book]
There's also a selection of other books you can read online [here].
Be warned that some of these books, such as Snake Wrangling for Kids do have chapters later on that focus on graphics programming using Turtle and other libraries. You may want to skip some or all of those chapters or focus on a more geometry based approach to working with them. Also none of these books focus on the more audio based forms of development commonly used in audio games.
Starting off they should focus on the fundamentals of the language with some of the beginner books, loops, variables, if/else, math operations, functions, and classes. After that they can start moving towards libraries like Pyglet or Pygame for creating windows, basic audio, key and mouse input, etc. A few key resources you may want to introduce them to would be Tolk for screen reader support, and some OpenAL Examples I have available for more advanced audio. Tolk has been experiencing some issues with its main download, but I have a backup copy [here] with a working example of it in use [here], and you can find my OpenAL Python examples [here], which include 3D positional audio, Effects, Filters, HRTF, and Recording.
If they have any questions or problems, feel free to drop a question here and we'll try and help out as best we can.
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BrushTone v1.3.3: Accessible Paint Tool
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AudiMesh3D v1.0.0: Accessible 3D Model Viewer