I use the task pane for this. Add a task to an individual or create a more general one. I do like the idea of colour coding though - perhaps the colour highlighting could indicate which people have tasks attached - and make it able to be switched on and off easily.
I would like to be able to show when a couple had no children - I can add a note but it would be nice if it would show in the main people view e.g. display "No issue" in the list of children.