Mandoon Estate in Caversham is rather kid-friendly. The Swan Valley itself has many fun things you and your whole family can do! Here's the link to Mandoon Estate's TripAdvisor page, and some links that can hopefully help you in your quest: https://www.tripadvisor.com.sg/Restaurant_Review-g2308113-d7214247-Reviews-Mandoon_Estate-Caversham_Swan_Valley_Greater_Perth_Western_Australia.html http://www.buggybuddys.com.au/magazine/read/top-10-things-to-do-with-kids-in-the-swan-valley-_416.html http://www.swanvalley.com.au/About_Swan_Valley/Family_Fun
Some great info here: http://www.expatliving.sg/Things_to_do/things_todo_news/Margaret-River-with-kids-Full-guide-to-family-fun-in-Western-Australia-78719.ece
Cullen Estate, has a garden, grass area and a toy box in the tasting room with couches for the kids. Xanadu, has nice lawn, playground and nice food