My family (2A and 2C) just went for 8 days trip in Perth (Mid June). We stayed 4 nights in perth and 3 nights in Margaret River. According to the locals, the winter is delayed in Perth. The night temperature is between 7-10 degrees at night and 14-18 degrees daytime. It is expected to get colder in July. My sons (7 and 9 years old) put on thermal, scarf and down jacket when we go out at night. Daytime, they only put on sweater and jacket.
Margaret river has many factories, caves and gorgeous beaches to explore. We stayed at Bussells Bushland in Margaret river, very clean and can see many wild kangeroos and birds. This is a bushland stay in a cottage. Hence, is an awesome experience for the kids to enjoy nature. We visited Wonky windmill farm at margaret river as they have a fruit orchard and a wide array of farm animals. The fruit orchard also allows the kids to try fruit picking which is quite interesting.
Perth, we stayed at Pan Pacific and FRaser Suites Perth. Highly recommend for Fraser Suites - very clean, good view and near to city.