I've always talked to kids like they actually have brains. I do the sing songy thing with babies because it just sort of naturally comes out, but with older kids I use the same sentence structure and vocabulary that I would with an adult (obviously not the same subjects though...). I've had people look at me oddly, but if my DS doesn't understand a word he'll ask or I'll follow a complicated word that I'm pretty sure he doesn't know with a quick definition. We do all the 4 year old play and silliness as well of course, but I openly admit to normally forgetting his physical age. I talk the same way to my 28 month old as well and he shows no signs of not understanding.
I've personally never understood why people don't give kids real answers to their questions. Why waste a perfectly good learning opportunity and a chance to earn respect by giving it? I know it bugged me when adults gave kiddie answers to serious questions I had.
I remember two times when adults underestimated my older son along these lines. Once it was my mother who was reading a beginning number book with him when he was maybe 20 moths old, I think? She was exaggerating all of her words and carefully counting the numbers. He was going along with it for the moment. I told her he already knew them, but she didn't listen. When they got to the last page and she was asking in a baby voice, "Do you know how many this is?" pointing to the one. He was over humoring her by that point and just looked at her, pointed to the numbers one by one and clearly counted without a pause up to 5. I thought it was hilarious.
The other time was when we were meeting with a possible teacher. She did the whole little sentences, singy voice bit and DS just looked at her with sort of an awe-stuck look of confusion. Needless to say we didn't go with that school option.
