Kid Friendly plot threads

[bayonder]

I have just started a game of Monsters and Other Childish Things with my brothers, ever since the short lived ICONS campaign I ran with them a while back they have been begging me to run another game, and since they are both major Poke'mon/Digimon fans I figured the Junior Monster Battle League would be a fun campaign to run with them. I've altered it slightly, all kids with monsters are allowed to pursue whatever career they wish within the law and most are at least technically member of the Junior Monster Battle League, a pay-per-view sensation that has been very popular ever since monster became known.The way it works is that as soon as two monsters intent to engage in battle is clear it is detected by special handwavium sensors at the nearest JMBL outpost were a special remote camera drone is dispatched and quickly reaches the battle field and begins recording, the winner of the battle is given a percentage of the money made from people viewing it, so taking on popular monsters or monsters whose primary profession is battle is generally worth more money.
What I'm having trouble with is trying to come up with kid friendly plot threads, I've planned out whole campaigns for this game but all of them deal with mature themes and are pretty scary by the standards of an eight and ten year old.
it's not just the scariness, it's the complexity as well, neither of them are very familiar with RPGs and the campaigns I have designed are pretty complex with all sorts of background events and mysteries to uncover, I've had adults who miss alot of the clues and paths to take. Mostly I chose the JMBL storyline to ensure the plot would be simple and familiar to them so they would have some idea what to do.

[Lord Xtheth]

I ran RPGs in a youth group quite a while ago. Granted the children in question were all a little bit older than 8 (they were ages 12-16) They still liked the games I had to run, even when things got a little "complex" or "Scary". You'd be surprised what kids can come up with in a RPG.
Alternately, if you realy are afraid that things are way too mature, or maybe a little off-putting for your audience I have a couple ideas:
Never underestimate the power of fairy tales: They're familiar to most kids, they're fun to write up as an RPG scenario, and the bad guys are usually monsters anyway. 3 little pigs, Red riding hood, & the boy who cried wolf all have a very monster-able wolf you can use. Jack and the beanstalk has a giant monster in the clouds, (I'm having trouble remembering all my fairy tales at the moment, but I think you get my idea).
Refference TV: You already brought up Pokemon and Digimon, take a look at an episode or 2, and if not copy outright what happened in the episode, then use a similar monster and story line, I'm sure if the kids saw the episode, or know the bad guy, they'd apreciate it.
Relate to kids: Kids deal with a whole different bundle of problems than adults, and it still effects them the same. Steal their character's mountain bike, Have the world's candy supply go missing, Have a monster take over their school and declare weekends as no longer existant!

I hope these were answers or suggestions that could help.