There are plenty of resources out there on what makes a quest story (HERE, for instance). Basically it’s the story of someone who must leave home in search of a person, place, or thing. The more compelling the reason they must leave and find this person, place, or thing the better.
Brigitta MUST leave the White Forest in search of Hrathgar or else her forest home (and all the family and friends) may be destroyed.
For kids we can simplify it into three major parts:
*Call to Adventure / Action ~ who or what MUST the character find?
*Trials and Tribulations (obstacles, setbacks, temptations, etc) that end in a MAJOR ORDEAL
*The Return (the protagonist comes home changed from the experience)
A fun way to explain a QUEST story to kids is that it’s like a ROAD TRIP. Most kids have gone on road trips, so they can relate.
Have your students outline a road trip they once took with their family. Have them look for the most dramatic, funny, scary, etc events from the trip and tell them to exaggerate some of the things that happened to them for effect (i.e. there was a huge storm that took the roof off the hotel, their little brother was lost in a swamp)
Take the reason they left home on this journey and make it REALLY important. They had to go to Disneyland to recover a stolen magical item. They had to go to their Grandmother’s house because her town had been over-run by dragons.
Change everyone on the journey into animal or fantasy characters (their Dad becomes a wizard, their aunt is a witch, their little brother is a goblin), BUT keep the relationships as they are. The Dad Wizard is STILL a Dad and refuses to use any maps. The aunt witch still always talks about how different things were when she was a little girl. And the brother goblin still likes to see how loud he can belch.
Write THAT story! Have fun.