For a family friendly and close to Perth location, it's hard to beat the Yanchep National Park. During the school holidays, rangers even present a variety of activities such as caving and koala watching just for kids.
There are wetlands and bush areas where walkers can hopefully see kangaroos, possums and bandicoots. Boats are for hire for paddling on the lake, there are picnic and barbeque areas and it's all within an easy drive of Perth.