Simple words, simple structure, simple ideas, basically simple everything. Also it would probably be a good idea to find a child and ask them about their feelings of depression (or just hurt and stuff) and see how they describe it, how they feel about it etc.
Also pictures are hugely important. Your story needs pictures, usually one big picture per page, and the number of sentences per page depends on the agegroup you are going for. This will probably be something that won't really be necessary until you've finalised the story, but you have to consider if you are willing to pay an illustrator, or who is going to draw them and for how much.
Quaxar said:
I suppose reading stories for children would be a good starting point.
Oh yeah, this is crucial, but don't just stop at reading them, take a pen and paper and write them out word for word as you read, this should give you a better understanding of how the author has structured their sentences and story, what words they use, how often they repeat important points, how difficult or easy the subject matter is, etc.
Finally, run it by a child when you've finished, or even several times during the process. That sort of input will be invaluable.