Writing for tweens and planning the ups and downs of the middle of the book. Should the ups and downs be in the same chapter, or separate? That is, one bit of action or challenge and the next in relaxed healing and growth. Then back to action!

It would possibly hinge on how you format your chapters. You could (a) separate your chapters into individual scenes, or (b) separate your chapters into individual sequences/endeavors to accomplish the story goal.

From what I have read, (which is fallible, of course) it is usually best to begin and end your scenes, and probably chapters, on opposing notes. Things are going great? A supervillain crashes the birthday party and kidnaps the cute three-year old. Things are particularly hopeless? That's when the traitor/henchman (that your hero has been so merciful to and has been stabbed in the back by so many times!) finally repents and reveals that important bit of info that has been needed all this time.

Doodle has excellent advice. Another thing to keep in mind is chapter length. It should be consistent throughout the book as much as possible, and if you're writing for tweens, I would also err on the side of shorter chapters.