Judge Orders Sia & Estranged Husband to Mediate Child-Custody Issues

No "Cheap Thrills" here ... Sia and her estranged husband Daniel Bernad have been ordered to enter a mediation session to try to hammer out the custody issues in their contentious divorce.

We broke the news earlier this week -- Daniel's emergency request for full custody was shot down by a judge ... but now, the judge is ordering them to get together for a mediation session over the custody and visitation issues for their 1-year-old, Somersault Wonder Bernad.

In court docs obtained by TMZ, the former couple is being required to at least attempt to work through the issues before the next scheduled hearing. They've got 2 options -- handle things through a video conference with a mediator ... or use the Los Angeles court system's online program meant to help parents formulate a parenting plan.

However, if they choose the second option and aren't able to reach an agreement, they're required to meet online with a mediator.

You heard it here first -- Sia filed for divorce in March after about 2 years of marriage, and the breakup has been anything but smooth.

Sia currently has primary custody of their son, with limited visitation rights for Daniel, in the arrangement between them, but Daniel tried winning Somersault back by accusing Sia of being a "danger" to their son, accusing her of struggling with substance abuse issues. Sia's celeb-trusted attorney Laura Wasser argued she's been sober for 6 months and has no issue being drug-tested.

The musician also claimed Daniel was the one reluctant to submit to drug tests, and reminded the court she has custody because he was recently the subject of an investigation by the LAPD and DCFS over alleged child pornography -- which is currently closed after he vehemently denied the allegations and suggested any evidence had been planted.