Actually I think chapter length does matter especially for MG, and shorter chapters tend be preferred. Also that's exactly how I do chapters, I do them once I"m done, not as I go. So don't think you are doing something wrong doing it that way. We all have what works for us. I do agree that you should look less at word count and more at the stuff CheG recommended. That being said, I would hesitate having chapters too much longer than 2K.
Number of chapters though is kind of irrelevant. It's about the general pacing of the book. Does it feel stop and start having that many chapters? Does it flow? Does it get tedious waiting for the end of the chapter? That's all. My first book that was 80K had 44 chapters in it. I have 26 chapters in my latest MG that tops out at 47K (btw, both sold )
I know these are old posts, but thanks for these helpful comments about chapter lengths! I'm working on my first MG novel (unless you count the incredibly juvenile one I wrote when I was 13!), and this is something I've been wondering about.