I can see Luna comforting her dad just as she comforts and encourages Harry to produce a Patronus (DH). If there is a funeral she'd probably pat a grieving close relative or family friend to the arm and say something like, "Oh, don't be sad" with the innocent expression of a nin year old.
Follow Azhure's lead on how she might say something extraordinary about her mum not having gone away forever.
Luna is described in the books to have 'straggly' and 'waist-length' hair. It makes me so sad to think that the possible reason for this that she didnt have a mother who would comb her hair every morning and tie or style them. Try to get this point across. Show Luna having trouble getting her hair brushed the next morning. She's a young girl standing in front of the mirror, pulling the brush through her hair, but unable to manage them. She's nine, but she's had her hair done by her mother every morning before, but not today. The tangles wont go. Luna is upset.
She approached Xeno, but the poor man has never done this once in his life. You could show Luna looking up at him expectantly, and watching Xeno's worried and concerned and surprised face.
You can show Lune creeping in the room where her mother died, and touching or looking around things that broke because of the spell backfiring. Luna might graze her hand over a broken something. Involve Xeno into it, and show him being a bit angry about Luna being not careful. Express Luna's confusion about the way her father suddenly behaved.
Xeno is in shock, and he is grieving. He also has the sole responsibilty of a nine year old child. He's scared and concerned.
But Luna cannot understand his state of mind. To her, like any other nine-year old, death just means going away, but not permanently. All you need is to express that confusion. Write with your hand, think with her mind.
Perhaps at night, you could show Luna having a nightmare, like sound of a spell backfiring. Luna could wake up, alone in her room and cry softly, hugging herself.
That made me sad. Seriously, its all about getting the emotions right. Choosing the right words and the correct descrption.
Hope I helped.