What is this thing called in example 3?
It is called a "Derrived table"

Is it a "mainstream" feature of Access that will be supported in the future?
It is a standard feature of ANSI SQL as far as I know and is very commonly used.

Where can I find documentation on it (this might be easier once I know what it's called)? 
Here is a blog article about them

In any case the query posed by Mitch is your best bet:

SELECT *   
FROM Families   
LEFT JOIN Children  
  ON (Families.FamilyID = Children.FamilyID) AND (Children.Gender="m")  
WHERE (Children.FamilyID IS NULL)


SQL Help in Access – Looking for the Absence of Data
I have a few pet wolves' in creative mode on minecraft (Pc) and one of them is on very low health (Its tail is sagging). Is there any way I can heal him?
