The characteristics of existing fairy stories are:
They involve a few children (to identify with)
They involve an adventure (that they can understand)
They involve a bit of magic (so anything is possible)
They involve goodies and baddies (cops and robbers, humans and bears/wolves)
The goodies always win.
So we need a story about children with friends (goodies) and enemies (baddies) who have an interesting adventure in which the goodies magic beats the baddies magic and everybody (except the baddies) lives happily ever after.
The story must start "Once upon a time there ....."
The most modern fairy stories I can think of are Alice in Wonderland and the Wizard of Oz. So that's the sort of idea you need. Best of Luck.