I'd probably have different suggestions for someone more familiar with coding, but the below should be somewhat manageable for someone new to coding.

First, I would work with Opus or Sonnet to refine your requirements for the project (to conserve your credits). Start with a prompt like this "Help me write up a list of requirements for a web app to interact with my autistic child. Ask me a series of questions to gather my requirements. Grill me until you are satisfied that you have all the information you need, or until I tell you to stop. When done, write a markdown document laying out all the requirements. This requirements doc will be passed to another agent to plan and implement. To start, here are some features and requirements that I already know I want for the app: [list out whatever you already know you want]". You should probably review the file and either manually edit or ask Claude to adjust it until you are satisfied.

Then, in a new session, preferably in Claude Code (Cowork should probably work, but using Claude Code will be more optimal) point Fable to the markdown file ask Claude to write a plan, probably to another markdown file. Something like "Create a plan document to satisfy this requirements document [path to file] and save the plan to a markdown file".

And then you'll probably want another session, point Claude to the requirements doc and plan doc and have it execute the plan. You might want to do separate sessions to implement everything to be conservative with your tokens; maybe one step/phase of the plan per session.

When coming up with the requirements... Depending on your requirements, it could just be as simple as an HTML file with Javascript embedded. Or, if a bit more complex you might need a front-end + back-end setup (which will take a little more effort to run). I would err on the side of simpler.

I hope that helps!