big words

If no one ever uses big words in conversation with children, how are they supposed to learn, let alone expand their vocabulary? I was having a conversation with my four year old niece at dinner this evening, and she was telling me about her friend's brother's tree frog. The word "rainforest" came up on my end of the conversation, but before my niece even had the chance to ask me what a rainforest was (and she would have too) my brother-in-law interrupts me to say, "why do you bother using such complicated words when you talk to her?" Hello??? I thought talking to a child, as an adult, you're supposed to keep them company and teach them things...to explain what they should and shouldn't do in the world. How are they ever going to know about things if you don't give them the opportunity to ask?
I just kept to myself. It wasn't even worth responding to. It wasn't like I was using words such as ubiquitous or parsimonious...it would take him too long to comment before he would even figure out what those words meant. I know, when I have children of my own, I'll make it mandatory for him to only use words larger than 8 letters when he talks to them. I'm trying to facilitate learning, and I get scolded. Yeah, what do I know, right?
Like many four year olds, my niece is pretty smart, but she's got to be given the chance. Kids are amazing, they are like sponges, easily learning two or three languages at a time. They love to talk, but adults refuse to pay attention... so many people have kids but spend no time devoted to their development. Children are an investment, but require guidance along the way. It's not like putting ten grand into a savings account, not touching it and getting a 2% return in twenty years. Well, my point is, we can't expect kids to know what we don't teach them, and we should have high standards for them, for they will strive to achieve, it's in their spirit to do so, especially when we can provide an accommodating environment.
k