Do all Australian aboriginal children have light colored hair?

No.

The Australian Aborigines established themselves throughout Australia, including Tasmania, long before Europeans "discovered" it. Small groups began arriving by sea at least 40,000 years ago; they probably traveled from Southeast Asia to Australia's north coast. Aborigines are dark skinned, with numerous regional variations in their anatomical features and hair color. Before the first European settlement of Australia, the Aborigines were estimated to number from 300,000 to 750,000. In 2006 approximately 517,200 Aboriginal and Torres Strait Islander people were officially identified; fewer than 100,000 of them are of homogenous ancestry, with the remainder mixed Aboriginal and European.

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In some areas, most of the children start out light haired, but go dark later. Staying blond(e) as an adult is not all that rare and is not necessarily due to Northern European genes entering the local gene pool. Blond(e) individuals were noted at the first contacts. https://en.wikipedia.org/wiki/Blond