"Wary" also means "openly distrustful and unwilling to confide"--both definitions fit the situation, because the risk of being reprocessed makes the man cautious about stepping out of line, and the questions make the man distrustful of the children's intentions. With nothing to hide or confide, the man is not completely unwilling to talk, but to protect himself, he decides to report the children.
“Now, my dears,” the words continued, “I shall of course have no need of recourse to violence, but I thought perhaps it would save you pain if I showed you at once that it would do you no good to try to oppose me.
"Insistent" also means "demanding attention"--both definitions fit the multiplication table, which is repetitively rhythmic to help young children memorize it, but IT is now reciting the numbers in order to focus the attention of the children and take away all of their independent thought.
The number words pounded insistently against Meg’s brain.
She tried looking at the mouth, at the thin, almost colorless lips, and this was more possible, even though she had to look obliquely, so that she was not sure exactly what the face really looked like, whether it was young or old, cruel or kind, human or alien.
not of natural origin; prepared or made artificially
“Of course our food, being synthetic, is not superior to your messes of beans and bacon and so forth, but I assure you that it’s far more nourishing, and though it has no taste of its own, a slight conditioning is all that is necessary to give you the illusion that you are eating a roast turkey dinner.”
"Bland" here is a synonym of "opaque" ("not transmitting light") and "vacant" ("void of thought or knowledge"), which are used in these descriptions: "The pupils grew smaller and smaller, as though he were looking into an intensely bright light, until they seemed to close entirely, until his eyes were nothing but an opaque blue" and "Charles Wallace stopped and turned slowly to look at Calvin with his cold, vacant eyes."
He seemed to be looking directly at Calvin as he spoke, and yet Meg was sure that the bland blue eyes could not see, and that someone, something else was looking at Calvin through Charles.
The Prime Coordinator does not like the negative connotation that Calvin has for the word "hypnotized"--according to IT, "for you, as well as for the rest of all the happy, useful people on this planet, I, in my own strength, am willing to assume all the pain, all the responsibility, all the burdens of thought and decision."
“But you’re being spoken through, aren’t you, just like Charles Wallace? Are you hypnotized, too?”
“I told you that was too primitive a word, without the correct connotations.”
Normally, Charles has a placid personality, which is strange for a five-year-old. Here, his placidity is strange because he can read and brighten his sister's moods, but under the spell of IT, he doesn't care that Meg is upset and pleading to see their father.
“Charles!” Meg pleaded. “Let me in to Father!”
“Why?” Charles asked placidly.
IT was the most horrible, the most repellent thing she had ever seen, far more nauseating than anything she had ever imagined with her conscious mind, or that had ever tormented her in her most terrible nightmares.
unhealthy vapors rising from the ground or other sources
For everywhere she looked, everywhere she turned, was the rhythm, and as it continued to control the systole and diastole of her heart, the intake and outlet of her breath, the red miasma began to creep before her eyes again, and she was afraid that she was going to lose consciousness, and if she did that she would be completely in the power of IT.