Question, do you literately want the child name to be “Project A- Meeting with B” or do you want the display expression to be this? Those or two totally different actions and in practice mean different things. Display Expression will not change the note’s actually name, just how it is visually presented, the note’s actually name will still be whatever you gave it.
I suspect what you’ll want to do is to an Edict that will update the Child name, you might try:
$Name=$Name(parent)+" - "+$Name.
Note, however, while this is a convenience for the name, it won’t help you with searches and organizing all the related notes to the parent. You could create an attribute, say $Project, and then do this:
$Name=$Name(parent)+" - "+$Name;$Project=$Name(parent);
Now you have the name that you can track against and an attribute that will also help you associate this note with the respective project.
There is a lot of other logic that you could wrap around this.