Wow, that's a massive trip! It's going to be great fun for the kids, and having a caravan would make it all a lot more flexible for you, which is probably good with three kids too.
The southwest of WA is lovely - there's lots to do and see for grownups and kids.
Hope it's a great experience for you guys. We used to do big family trips every now and then, including road trips, and they're memories you keep with you always. I'm sure it was hard on our parents sometimes (three bickering kids in the back), but I have great memories of the trips.