I want to talk story structure/the craft a bit

I dunno, I'm seeing what you want and thinking what you want isn't what you're going to get . I mean there's nothing wrong with quiet scenes, and scenes with a bit of exposition and dialogue that develops character, but ultimately in middle grade you have to keep things moving. You just have to. So I think you kind of have to get over that desire to not do both things at once. I think you instead should embrace the challenge.

Also, I mean, THE GREAT GATSBY was 47K and that had plot and was rich with character development and world building. There are a lot of adult novels that manage to be short and yet still complex. 60K for a middle grade gives a TON of wiggle room. And heck my first published MG was 80K - the length of an average novel. So basically I think one can absolutely do a lot in a short span of time, but I also don't think MG is as limited length wise as you think it is.

The most important thing I think is "Will kids enjoy this". We don't have the luxury adult writers have of a patient audience. Of a reader who will stick with you even if the beginning doesn't entirely draw you in because they want to give you a chance. Our audience has nothing to prove, they won't read a book just because it's an award winner (their teachers might suggest books just because they are award winners, but actually reading cover to cover? That only happens if it's also awesome). Backstory, careful slow character development, none of that holds the interest of kids in and of themselves. But combined with solving a problem, or journeying somewhere, or rescuing a trapped animal, then that's of interest.

And honestly, I feel that also holds true with adult books too quite frankly.

I will give you this tip: how a character responds to something tells you a great deal about their personality, and even their backstory. So instead of possibly just describing a view or a place or a house or whatever, have it filtered through the perspective of a character. What does the character think of the view or the place or the house. Most MGs these days are third person limited so that gives you a lot of opportunities to do just that. Kids have opinions. A lot of them. Just like us. And opinions reflect our beliefs, our mental state, our upbringing. It's an excellent way to show not tell a personality.

Anyway, yeah, not sure this helped or not but there you go.

I'm an adult who sometimes reads MG. If a scene isn't moving the story forward, I get bored.

As an author, if there's part of a narrative I'm driven to write, but the scene feels tangential, I've generally needed to step back and assess whether the scene is getting at something central, which I need to develop more--or whether the scene is part of a separate, second story I want to tell.

Hmm... there are different kinds of MG novels (just like in any age group) to cater to different tastes. There are "quiet" ones that have tons of character development that are just as page turning (to me) than the action action action type. Even in quiet novels, things are always happening, though they're not the explosive fireworks kind of action. I've been leaning towards reading and writing quieter novels lately. Maybe u just gotta decide where on the scale you want your story to be.

I think you're seeing this in terms of a false dichotomy. Plot and characterization are not separate things; they're two sides of the same coin.

For example, imagine two versions of a story about a woman whose only child is killed by a drunk driver. In version 1, the woman is devastated. She falls into a deep depression, refused all attempts to help her, and cannot assimilate the fact that others too---starting with her husband---have suffered a great loss. The marriage falls apart, and because the woman is absent so often, she loses her job. Eventually she loses her home and ends up on the street. Her life is destroyed.

Version 2: A woman loses her only child to a drunk driver. She is devastated, as is her husband. They turn to each other for support and they join MADD to advocate against drinking and driving. She accepts the comfort of friends and struggles to continue working. Eventually, while they never lose the sorrow, life regains some of its flavor. Through shared suffering and the kindness with which they treated each other, the parents are closer than ever. They go one to have three more children and a full life together.

The same precipitating event starts off both stories: the child’s death. The different characters of the two mothers affect the events that follow, and those events will impact on the characters of the two women, changing them.

Chicken and egg.

Omelet, anyone?

Simple answer, I do what serves the story.

It's not about me and what I want. It's about the story I'm telling (showing) and the best way to tell (show) it. Audience is a factor in that. (Try selling a middle grade coming of age plot to a two-year-old.) But there are many other factors too: plot, characters, setting, word choice, narrative voice, pov. They all have to serve the story.

As others have hinted, it depends...

Basically what Toothpaste said, but for me, the great thing about MG is that all the fluff has to go. Generally in MG major character evolution is not often sought. Characters tend to 'do', and in doing, they reveal who they are, and their moral codes, which that age group appreciates.

Character evolution or self reflection/actualization is usually YA or Adult. (Usually)