My wife and I do the same, and the results have been great. An old friend of mine visited me and met my daughter for the first time when she was two and a half, and she just walks right up to him and says, “Hello. My name is _____. It’s very nice to meet you.”

When my current two-year-old is in a bad mood, we’ll ask him if he’s being a curmudgeon, and he’ll say “No, I’m not being a curmudgeon.” They speak in full sentences because my wife and I speak in full sentences. They use big words because we use big words.

On the other hand my daughter is five now and still thinks it’s pronounced “breafixt” instead of “breakfast”, and we don’t correct her because it’s adorable. So we still have fun with it.

I don’t think any of this means they’re geniuses or are guaranteed success later in life or anything. They’re probably both gifted, but that just means they’re a couple years ahead. A four-year-old who talks like a six-year-old is a great parlor trick, but a twenty-year-old talking like a twenty-two-year-old isn’t going to give them a big leg up. That’s why I like to get all my bragging in now.