I'm not a novel writer, I do PBs, but generally speaking, I wouldn't worry too much about the market. Books take 1 - 3 years from acquisitions to hit the bookshelves (at least PBs do) after months/years of querying. So writing to the market typically doesn't work very well. And if the book is unique and "un-put-down-able" and an agent or editor loves it, they might just try even if the market is saturated.
With that being said, I hope other MG writers chime in. As I said, I don't write or generally read the MG market, so I'd hate to give you bad advice.