Sometimes the hardest thing about writing is starting. My kids have struggled with this process at times, and to help them I pulled from all those things I learned from college, books, and writing conferences. Maybe these ideas can help your kids too--or yourself, if you're a beginner.
First thing you need to do is ask yourself:
1) Who is this story about? You need a CHARACTER. It might be a girl, boy, baby, dinosaur, mouse...you get the idea.
2) Where does your story take place? Is it in outer space? A forest? A high school? A war? Is it in the future? A long time ago? All of these things are part of your SETTING.
3) Now that you have a Character and a Setting, you need to narrow in. What is going to happen in your story? What exciting things will your Character encounter? You need a PLOT.
Then comes the fun part. I love asking my kids to make a list of things about their character's life, or just to tell me about them. The first time I did this, my 12 year old said, "But that's not going to be in my story. So, it doesn't matter."
It does matter!
The information may not be directly in your story, but it is indirectly there. It shapes your character's decisions, it makes them more real, and your story will be much more interesting. It might also help shape your Plot. I do this with all of my characters, and the earlier in the writing process, the better.
So, ask some questions, jot some notes so you remember.
How old is your character?
What do they look like physically? Are they short, tall? Do they have curly hair, long hair, purple hair? Big feet? Crooked teeth? Sparkly skin?
What do their parents do? Are they doctors? Pirates? Darth Vader?
Who is in their family? Are they the 13th son? Are they an only child?
Who are their friends?
Who are their enemies?
What is your character's favorite food?
What personality traits do they have? Are they obsessed with food? Do they love dogs? Are they afraid of paper cuts?
****And the most important questions to ask about your Plot and Character...
1) What is their goal? What is it they really want? What are they going to try to achieve during the story?
2) Why do they want this? Is it because if they don't their mom will die? The more the character has to lose, the more believable the goal will be.
3) What is stopping them? This is called Conflict. Is it Voldemort? Is it no money? Is it a horrible storm?
If nothing bad happens to your characters, nothing interesting will happen.