Sia Wears Matching Halloween Costume With Son Amid Custody Battle

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.

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 husband, Daniel Bernad.

As you know ... Bernad filed for emergency custody of Somersault -- alleging Sia suffers from substance abuse issues and is therefore a "danger" to their son. In return, Sia -- who is being repped by Disso Queen Laura Wasser -- claimed Bernad was previously under investigation for child pornography. Bernad has strenuously denied the allegations and suggested any evidence may had been planted by the singer.

A judge denied the emergency motion and two have been ordered by a judge to meet with a mediator to try and hammer out a custody agreement before their next scheduled hearing.