probably a bit close to Perth for a free camp. Willeton is not too far from Fremantle where there are a few caravan parks but it if its holiday season you may need to book. Fremantle will assure you of cool breezes. Mullaloo is quite a way further north. Finding a dog friendly park may be a problem. There's a park at Ascot and one at Karrinyup that might suit  - not sure about dogs. Might increase your options if the dog could perhaps stay with the families you intend visiting.