I strongly suspect that what's going on with "people who talk to children like they're adults" is that they talk to children like they're people.

Strongly seconded. I've long been certain that this is the reason I get along so easily with that kids older than about 4. I listen to what they're saying, ask for more information about things they're interested in, and enthuse about things I'm interested in. In general, I talk to them like I'd talk to friendly acquaintances.

From what I recall of childhood, people who aren't obviously disingenuous towards kids are rare, and precious to kids.