When Nick Reiner was photographed alongside his family at the Los Angeles premiere of his father’s comedy musical, “Spinal Tap II,” in September, he wasn’t smiling. Instead, the son of legendary Hollywood director Rob Reiner and producer Michele Singer Reiner stood impassively for the family portrait alongside his siblings.

After the killings of Rob Reiner and his wife, Michele Singer Reiner, their children are speaking out for the first time.

In addition to two counts of murder with a special circumstance allegation of multiple murders, Reiner, 32, also faces a special circumstance allegation of using a deadly weapon, according to court do...