In Skyward Sword there is a side-quest where Link has to help locate a child who has gone missing. This page is devoted to helping you complete the Lost Child side-quest.
Lost Child Quest
After you return to Skyloft with the Ruby Tablet, you will notice a lady outside of the Statue entrance with a speech bubble over her head. If you talk to her she will inquire to to help locate her lost daughter, Kukiel.
The next thing you should do is talk to some people throughout Skyloft. You should find out that an old man at the Lumpy Pumpkin believes in monsters in the graveyard. So you should head to the Pumpkin Landing and locate this man.
So fly to the Pumpkin Landing and you will find the old man at a table inside the bar. If you have not started the Skyward Sword Pumpkin Harvest Quest then you can also do that.
Talk to the old man and he will tell you to push the gravestone nearest the tree at night and a shed will open up.
So fly back to Skyloft and do exactly what he said to do. You will need to find a bed to sleep in to turn day into night and then head towards the graveyard. Slash the tombstone with your sword to activate it and you can then push it back. The nearby shed will now open.
Climb down the ladder and work your way to the old hut. Inside is a demon named Batreaux who wants to turn into a human. This is a totally different quest which will be covered in another guide.
Kukiel will also be there. So talk to her and then exit and head to the girl’s house, near the bridge and Bazaar. Sleep until morning and talk to her mother to get Gratitude Crystals.