Write Away

Could you be more specific as to what you need ideas for? Plot points, character traits, setting? So far we have a character with three traits (age, gender, ability [including no powers]) and a parent with an unfinished trait.

Setting basics: Is this within the Frozen setting, or one like it? Are you borrowing the plot for new characters/setting? What year is it? Where in the world is it? Rural, city, etc? Rich, poor, one of the middle class? I'm sure there are other things, but you'll end up finding them.

How severe is the girl's autism? Is this a story about autism itself? About a parent's struggles with raising a child with such a difficult disorder? Is it just there to be an internal obstacle for the main character? How does she feel about her autism? What do you know (or how much time are you willing to spend learning) about autism? Autism covers only a few bits about a person's personality and life. Fill in the rest of her self.

Start off easy: What does her bedroom look like? (I like the bedroom and the kitchen because it lets you build both character and setting!) Does she have nice, soft pillows with smooth sheets and a heavy blanket, or does it all scratchy and cold? Is it ordered, clean, cluttered, outright dirty? Is her rug bearskin or polyester? What does her window look out to? City, forest, farms, the cute stableboy next door? Is she watching pigeons or squirrels? Does she like looking out the window? If not, why not? Does the world suck, or does she just not like what the view provides her? Maybe she wants to improve what she sees (clean up the streets, replant the forest, stop people from fighting, whatever fits)!

You said she only gets out when her mom can't leave her at home. Why does she leave the girl at home: overprotectiveness, neglect, fear, outright abuse? If so, what caused that reaction? Did something happen, is she afraid of what might happen? Where is the girl left? Does the mom trust her at home, or does she call over the elderly neighbor lady just in case? Where's the dad? Did he die in the war, scarper off with another woman, not know about the child?

If you can't tell, I get a lot of my story written by finding answers to these kinds of questions. I often find the plot via character-/world-building and vice versa. Also, I've not actually seen the movie, so I would be the opposite of helpful in that regard.