In the poem "Kid Stuff" by Bruce Dawe, is the reader a part of the audience?
While "Kid Stuff" definitely addresses the poetic persona's father (the persona in a lyric ballad like this may sometimes be presumed to be the poet though this is not always the case). The father being spoken to in this sort of address in a poem is the implied audience:
Returned from timber cutting up at Orbost
To our little single-front rented place in Dickons Street
-Comically muscular (like a health studio ad)
From axemanship and boxing in the camps.
The reader is always the expressed audience, the audience intentionally addressed. Therefore, yes, by poetic convention, the reader is the intended audience while the interior character--in this case, the father--is the implied, suggested audience.
"Implied" means strongly suggested but not directly stated or indicated. "Expressed" means manifest, set forth, represented.
Understanding who the implied audience is (e.g., father) and what the relationship is and the motive behind the communication is will help the expressed audience--you--to understand the meaning and import of the poem.