Okay kiddo, so imagine you have a new toy that you really like, but you have to share it with your little brother or sister. You might make a rule that says they can only play with it when you're not using it.
Well, a long time ago, Australia was kind of like a big toy that belonged to the "big brother" of the British Empire. But over time, Australia started to grow up and wanted to be more independent. So, they made a rule called the Accession Declaration Act in 1910.
Basically, this act said that even though Australia was part of the British Empire, they were their own country and could make their own laws without having to ask the "big brother" for permission. This was a big step toward Australia becoming the independent country it is today.
So just like you can make your own rules for your toy, Australia was allowed to make their own rules for their country, all thanks to the Accession Declaration Act in 1910.