What _seems_ to be happening here isn't that the big companies (like Tyson) are flagrantly violating labor laws.
Instead, they hire contractors for some things, like cleaning. Those contractors start out by hiring illegals with false documentation - which, as I've said before, is defensible as "we didn't know".
Those contractors are small businesses themselves, and their labor practices loosen over time.
Maybe it starts out relatively understandably: someone brings their kid with them and the kid is doing some of the work assigned the parent. Once they've done that a while they (the kid) applies with false documents in hand, already know the job well, and live with other employees of the company so they're likely to be able to make it in.
That's still plausibly deniable by the company. Yeah, they know the kid isn't here legally, and they at least strongly suspect that they're a minor, but they don't really have legal proof of it.
Other families are seeing that one earning income, so they get their children false documents and have them apply. Because this is one of the few places where minors are employed - and both the minors and their parents are _wanting_ the minors to be employed - they have an influx of cheap(er) labor. Wages drop, the adults leave to work elsewhere, and now we have a company employing almost exclusively minors while doing work for a major company like Tyson.
If I had to guess, I'd say that the parents mostly end up working for Tyson while their minor children work for the various contractors employed there.
Finally, I'll point out that "minor" and "child" are basically synonymous from a legal perspective, but not from a social one. It's not at all uncommon for people where I live to work full-time over summers starting at ~14, and part-time during the school year at ~16. That's 100% legal here. In fact... my state offers "hardship endorsements" for driver's licenses that allow minors as young as 14 to drive on their own to and from school or work.
People's lived experiences differ hugely. My oldest daughter is 14, is homeschooled, and is literally begging me to let her work. There are very limited ways I can make that happen at her age, so we're focusing at the moment on entrepreneurship. When she turns 16, she'll have an (almost) unrestricted driver's license and be eligible to legally work lots of places with some limitations. She'll likely end up working in either food service or retail at that point.
I was hauling hay in the summer from 12-16, and that seems way more dangerous than cleaning a slaughterhouse - and yes, FWIW, I've also worked in a slaughterhouse, so that's not a completely uneducated assumption. Hauling hay is mostly a matter of walking/jogging alongside a truck or tractor pulling a trailer through a field, picking up ~50# hay bales, and throwing them up to 10' or so in the air to someone on the trailer to stack them. Once the trailer is full you get to go to the barn, unload them one by one, and stack them to the ceiling. All of this is happening in full sun and ~90-105ºF, and while wearing blue jeans and long sleeves (often flannel!) to protect yourself from the hay. You will absolutely experience heat-related health effects, up to and including passing out in the field if you don't actively stay hydrated.
Given that the above is the status quo for most families here, I would have no problem with my daughter choosing to hose down and clean a slaughterhouse floor at night instead.
So.. yeah. "Minor" doesn't mean "child" in common parlance, and where "child" ends and "young adult" begins varies significantly between region and culture.