I'm so sorry you've experienced such awful treatment in your time here. Do you think it's a localised problem with the specific area you are living in? Have you looked elsewhere in WA?
I can't speak personally on the subject of racism or mixed race families, but our neighbours (from the UK) are mixed race and have 3 wonderful children and as far as I know have not experienced any of the type of attitude towards them that you have in the time since they arrived in WA earlier this year. ( I say as far as I know, as they only moved in across the road from us last month). The boys who attend the same school as my children are very settled and happy and have alot of friends.
I have seen a few other mixed raced families here, (who have mutual friends in common) along with indian, sudanese and many other races of families living here in our suburb and they are all happily settled and experiencing non of the problems you have.
I think some aussies can be likened to the "OAPS of the UK" in their terminology... my own mother refers to black or asian people as coloured and I HATE it and always tell her off... she says it's her age (80) and that's how she grew up and that she isn't being offensive... doesn't mean she is racist, just means she is a bit blummin ignorant and maybe that's some of what you're experiencing?? Some say that Australia is about 20 years behind the rest of world with some things..... and in others they're still about 40 years behind..
I hope your experiences don't have a lasting effect on your family and that the kids, wherever they are find some lovely friends who deserve their friendship.