As JH- said, it shows mutual understanding. In this particular case, it is a children's story, and you'll see this a lot in children's stories or songs. In some cases it's because you're retelling a well-known story, so they will know this bit already - but in other cases, even though the listener/reader doesn't know the story, using this phrase draws them into the story, making it seem more familiar. It gives it a feel of a familiar, commonly-told tale (even when it's new!)
Especially in this story, the sentence describes what the children are doing on Christmas Eve, and it's a familiar action, that the reader will of course understand and agree with - even though we're reading this story for the first time, the actions are familiar and expected, so using the -지 ending draws us into this story, making it more familiar to us.