Lovely interest, indeed. 

Character ages would range between 8 and 15. A diverse representation is favorable, so that everyone does not choose the same age. The characters do not necessarily have to be friends before this story. However, they can if you want them to. In any other case, they just live on the same street and they know about one another.

As for size of the cast, we'd look at something manageable such as 5 or 6 total.