The side quest in Skyward Sword, Missing Child tasks Link with finding the lost child of Kukiel, after she goes missing around the graveyard.
Skyward Sword Missing Child Side Quest Guide
To begin, talk to Wryna once you place the Ruby Tablet inside the Statue of the Goddess. You will see Wryna looking for her daughter outside the statue's room.
You now have the option to talk to Orielle and Rusta, who will offer you clues on Kukiel's whereabouts. Whether you decide to speak to those two or not, continue the quest by heading to the Graveyard at nighttime. Any bed in Skyloft will let you rest there until night if it's currently daytime.
Now that it's nighttime, head over to the Graveyard. Inside here, there will be a tombstone under a tree, strike this tombstone and it will start to glow, allowing it to be used as a lever. Give the now-glowing tombstone a pull, and the storage shed that Rusta mentions had a monster in it will open.
Entering the shed will have a cutscene trigger, showing a demon that has been hanging out in the shed, this is the monster Rusta mentioned. Trying to engage in combat with this seemingly hostile character will trigger another cutscene in which the demon introduces himself, asking for your hand at collecting some Gratitude Crystals.
Now you can sleep until morning and head back to Wryna to give her some closure on all of this, completing the sidequest and earning you five Gratitude Crystals!