Since the content of the story is unknown, the ending can be humorous in several ways. The ending can be unexpected with a twist of the plot and end with someone other than the expected character being the one to cause trouble. Perhaps a new character enters at the last minute to perform some magic trick which can be taught to the children to rescue the child in need. Perhaps the listening children can sing or dance at the end to perform the ritual require to rescue the duck. Children love to move and depending on the age for which the book is intended, I would use made up words, created songs, or silly dance moves to show children that humor can be composed of many ideas.