Parents are notes
Write in the parent like any other note. Its children show in its margin and under its text.
On disk
A note beside a folder of the same name is its subtree. No hidden index, no special format: open the folder in Finder and it makes sense.
Move the whole branch
Move to… or drag a parent and everything under it comes along. Drag a note onto another to make it a child.
Renames follow
Rename a parent and its folder is renamed with it, so the children stay where they belong.
Try it on your own notes
Free during the alpha · macOS 14 or later · Apple silicon · 10 MB