Hmmm... lotsa things already said here... just some thoughts:
If you want the PC's to be kids, make it such that only kids are still 'fresh' enough to be able to see and interact with the spirit world, adults simply have no clue and cannot see them. A threat to the 'Real World' by some sort of invasion by fairies can therefore only be stopped by children...
If you want to stay away from goryness, blood and guts, have the 'spirit world' fairies sorta 'project' into the Real World, so that vanquishing them basically only banishes the spirit's 'avatar' back to the faerie realm...
Definately have a look at cartoons, especially lately, there's been several about kids with magical powers and such which can also be tons of fun...
Have a look at the old TFOS (Teenagers from Outer Space) game, then convert some of the ideas into a more fantasy based setting, should have plenty ideas to mine for how to go about such stuff...
I think it is important to think about the age categorie you are shooting for, as I think there'd be a very distinct difference between pre-teens and (almost)teens in their tastes and likes as far as settings and adventures go...