My parents always spoke to me as an adult. I'll leave it to you to judge how well it turned out.
That is, except me when I was six... My parents would go to a school meeting and the teachers and other parents would be surprised -- "Oh, you don't speak in written language like your son?".
I guess I had been spending quite a bit more time with books rather than people.
I've sometimes had the same problem. my native language is French, and I mostly learned English by myself with books, computers, and music. Sometimes when I spoke, it sounded more like something a character in a book would say than what a typical person would.
Exposure to more flesh-and-blook Anglophones has mostly fixed that, though.
I've received numerous compliments on how well my children speak. All I ever did was speak to them like people (in the way that I would like to be spoken to), and explain any concepts or words they didn't understand using concepts and words they already did understand.
It just seemed...obvious. If you're going to treat your children like they're idiots, why would you expect them to turn out any different?
And now at twelve he gets compliments on his articulation and engagement, and he treats me as a normal human being with all rights and privileges. Although I expect that treatment to stop any day now, as he approaches his teens. Nice while it lasted though. :)