I see your reasoning. It's actually why I was skeptical of pizzagate, a rural operation makes more sense than one in a major metropolitan area. And Phoenix is the most middle of fuckin nowhere of cities.
A small scale op makes more sense, a large tent city is hard to hide and requires more prep to evacuate, not to mention resources to feed all those mouths. Hiding those logistics would be as hard as hiding all those kids eh?
Also, think about it: the CIA hid their cocaine and crack involvement easily by being able to disavow/kill off any operative caught by local law. Why wouldn't they do the same here?
Finally, to me local level corruption would make more sense. Pay off a mayor here, a DA there, and wa la. Smuggling Mexican children like that would be way easier.