-A girl has the choice of going trick-or-treating or staying home and keeping her sick younger brother company. She really wants to go have a good time with her friends, but she doesn’t want to make her bro sad. You decide the ending.

or:

-A girl promised her friends that she’d make their Halloween costumes, but she waited until last minute and she’s not sure that she can get done in time. She doesn’t know whether to rush herself or just buy some costumes from the store and pretend she made them herself. Again, I’ll let you decide the ending yourself.

or, something a little less moral-dilemma-ish:

Some kids have a contest to see who can collect the most Halloween candy, so they split up into two teams and go for it.

Good luck!

Or, what if you wrote from the perspective of an elderly man or woman handing out candy, and have them come to some sort of realization as the night goes on and kid after costumed kid comes to their door. Maybe they are missing their own grandkids and are depressed, or maybe their kid died in the war or something…..