While Bartholomew Cubbins is rarely afraid of speaking his mind, he's certainly got his fair share of anxieties in Bartholomew and the Oobleck. They crop up mostly when Bartholomew feels powerless and obligated by his duty to the King. Meanwhile, he knows that the King's selfish behaviors are leading the entire kingdom into disaster, and he's unsure what he can do about it except race around and find more incompetent adults. Wouldn't you have a little fear too?
Fear is also implied (fancy, we know) when Bartholomew bites his tongue or changes his behavior after the King has gotten angry at him. And fear is probably a healthy thing to have, too, because the King won't change until he's good and ready, and there's only so much Bartholomew can do to move that process along.
Q: Why doesn't Bartholomew do more to stop the King from destroying the kingdom?
A: He's trying, okay? It's tough to keep on keepin' on when your boss tells you to can it.
Q: Why is Bartholomew so worried about what the magicians are doing in their cave?
A: Did you see those guys?! They've got creaky eyelids. Plus, Bartholomew knows that what the King needs is a personality change, not oobleck, so at the very least, whatever they're doing can't be helping matters.
Q: If the adults are so scared of the oobleck, why don't they do something about it?
A: Because they're incompetent, duh. And because the King's change of heart is the only thing that can stop this beast.