Use this while talking together. The child supplies the ideas. The grown-up types. There are no wrong answers.
Give a few choices, but leave room for a completely new idea.
Ask him to draw it on paper first if possible.
This turns a character or idea into something playable or interactive.
Optional. Don't force an educational mechanic if it makes the idea less fun.
Capture things he says that don't fit anywhere else. These may become the best bits.
Generate a clean summary to paste straight back into ChatGPT or Codex.