Lone child nodes cry out for siblings.

When I was in middle school a teacher told me only to put a sub-bullet point in an outline only if it wasn't an orphan (if you had one sub-point it should have at least one sibling, otherwise don't include it). This was miserable advice because it ended trains of thought which might otherwise grow into something.

On the other hand it could be better framed that if you have only one child, you should brainstorm to come up with others.