I’m an aspiring author with one magazine credit and a fabulous agent. I’ve recently had some good news, an editor from a major publishing house is interested in my manuscript (the joy, the terror). She’s currently circulating copies to her colleagues and I’m waiting for news and trying not to hold my breath. I’ve had a lot of positive feedback from other editors (via said fabulous agent); however, the outstanding concern is that the manuscript might not be long enough for the age group (I aimed it at 8-12, though some publishers felt it would be better as a 9+). The manuscript is 32,000 words long. I do know this is short, however it felt like the natural length for this specific story. It’s a long preamble, but I would love an editor’s honest take on this situation. Is a short story just too costly to print? are they unpopular/unsaleable? or are they trying to let me down easy? I’ve indicated that I’d be more than happy to write more (provided it was more that added to the story, not just more words to bulk out each chapter), and that I would be thrilled to have an editor’s opinion/brainstorming power to help me with some ideas. Besides that, is there anything I can do?No, that's the right approach to take. In your position, I would assume that those editors who simply rejected it on the grounds that it's too short didn't have any particular ideas (or at least, ones they wanted to share) for how it might be longer.
It's possible you will find an editor who will be willing to publish this at its current length (it's on the short side, but I can think of shorter things published for middle school), or you will find an editor who's interested in brainstorming ways to make it longer.
If I were you, I would be thinking hard now about how you might make it longer (in a way that, as you say, serves the story rather than just padding it). If you're feeling stuck, ask your agent what she feels are the primary strengths of the manuscript and then concentrate on what you could do to develop those more.
Waiting and worrying are probably the couple of worst things for authors, but the good news is you have something to do that prevents both of them! That is: writing.