My kids really want me to run a role-playing game for them. Sadly, I just have no inspiration at all for a world, a plot line or even a character. They like witchcraft, secret societies and a little bit of horror. It seems like there should be an awesome game in that somewhere but it’s not hitting me. So what do you do when you need to drag up inspiration from somewhere? How do you come up with something?
Conversations, statements, body posture, inanimate objects, asking what if. Something happens externally that triggers a thought process internally.
How old are your kids. Young kids don’t care about character development or story arcs. You can get away with deus ex machinas, cardboard characters and holey plots more easily.
How about a secret society of demons that is bent on destroying the world and a trio of witches from different schools of magic must band together to banish the society to the gap between dimensions before they attain the artifact of power and begin a reign of terror.
Alternatively the reign of terror has already begun and the witches must steal the Artifact and return it to its resting place to awaken the gods so they can deal with the demonic intrusion.
It’s not exactly new or original but one could have fun in such a setting, I should think.
Ask yourself, “what if?” and “why…” questions, that’s pretty much all there is to it. Start with what you know you want to include from your OP… then ask a what if and follow it with 3-5 why questions. Rinse, repeat.
For inspiration, check world history. Search around for what others have done in other stories or games. You can dramatically change a story line with a few simple tweaks and changes and make it fresh.
Many famous books started from adults making up stories for kids on the spot.
Often, they would base the stories off the kids (without the kids realizing it) and make up great fantasy worlds inhabited by their kids alter-egos.
If you have, for instance, a child who is afraid of dogs you would have a character who is the Canine Prince, with a band of clever pooches who rob from the dumpsters and bring food to the starving dogs in Woofville, governed by the evil cat, King Pussyfoot…or if you have a daughter who is shy, the story would be about the quiet Princess Hush who never said a word by day, but at night she would dress as a commoner and go talk to all kinds of strange animals and plants who only spoke when spoken to!
Depending upon the ages of the kids - playing off their fears and giving the characters courage and special abilities makes them ask questions and get involved. Just envision what the kids would have fun doing if they could face their fears and overcome them - and go from there.
Running a roleplaying game, especially for kids, you can totally get away with copping the plot - or, at least the plot-hook, since if the kids are into the roleplaying aspect at all, they’re likely to derail you if you try to stick to a pre-designed plot too much - from a book or TV show.
I get a lot of my ideas for what to write about from Cooks Source magazine. 
<harlan ellison>There’s this little store in Schenectady. Every month I send them a cheque for $25 and they send me a bundle of ideas.</harlan ellison>
In a bar down the street. There’s a machine in the back.
Seriously, here’s a trick. Steal themes from something else. Like a musical album. Use the lyrics to free associate a storyline, map, what have you. It’s still hard work, though.
Beaten to it damnit!