It's not necessarily about "sentience." Remember that those from farms akin to Grace Field are a minority. The majority are the mass produced humans who can't function. The ones who probably don't even know where they are, have been plugged in for their whole lives, can't talk, can't do anything but live their whole lives suffering until they're ready to be eaten. So whether they can think for themselves or not, it doesn't really matter.
And while monsters can't be exposed to the Grace Field like farm produce, they can to the mindless ones. And I don't see why, if we're thinking of them as a human-like society, the parents wouldn't bring the kids to those farms to see what's up. Why the kids wouldn't be taught about this in their own schools. Why the parents wouldn't come home, have their kid ask them what they've been doing, and the parent would just give a straight answer, where the kid—as they do—would ask more questions about the animals they're guarding. If they're supposed to be as smart as we are, I'm not too sure they wouldn't be asking questions or thinking about what they're actually eating.
The only thing I can see being kept secret is the fact that the kids need to eat humans to function. But there'd be no reason to keep this secret, either, when it's literally what is needed of them to survive. The "children" will grow up to be just like the adults. They're not some next generation who will end the cycle of suffering that's been going on for however long it's been.