Hi! Thinking of planning a farmstay too! This are my "research" thus far for some inspiration. Thought to share as well. :)
http://www.petitetravellers.com/2014/08/harvey-and-bunbury-farm-stay-and.html
Her suggested itinerary looks fun and interesting
http://www.wheressharon.com/australian-travels/hanging-cows-sheep-farm-stay-victoria/
The airbnb place recommended sounds like a good choice!
http://www.expatliving.sg/Things_to_do/things_todo_news/Margaret-River-with-kids-Full-guide-to-family-fun-in-Western-Australia-78719.ece
Short writeup but the suggested activities looks promising..
http://sg.theasianparent.com/child-friendly-farm-stays-in-perth/
some farm stays potential candidates...
Sorry! Didn't have any personal experiences (yet!) but hope these would help a little. :)