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Sia and her son looked ready to color in the Three Little Pigs together ... matching as a pair of pink crayons while going Trick-or-Treating amid a vicious custody battle.

The singer-songwriter took her one-year-old son, Somersault , around their Los Angeles neighborhood Friday ... and, the two dressed in their bright pink outfits -- with the little dude sitting up in his mom's arms. Backgrid

Sia looked happy while out with her kiddo ... smiling wide in multiple photos -- and, Somersault got off his feet while rolling around in a stroller which was pushed by a man who appears to be his uncle Jon Bernad -- his dad's brother.

The night out for Sia and her son comes at the end of a long week for the star as she's been wrapped up in a custody battle with her estranged hus

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