Im really into greek mythology and wanted to make a short story about cerberus HELP?

for those of you who do not know. Cerberus is the gaurdian of the underworld in greek mythology he is the 3 headed dog who does not let anyone escape from the underworld or come in. if you try to you will be eaten to shreads. i want to write a short story about this mythical creature. The audience of my book is intended to be around 8-13 years of age.what should my story be about?