My DD loves pin trading. She's 8, but did her first trade when she was 6 or 7. I would advise starting her off trading with CMs. They're friendly about it and that should be a comfortable way to get her started. I've found that the interaction between DD and the CM is best when I take a back seat and it's obvious that I am not pushing her to pick certain pins.
While most pin traders we have encountered have been wonderful people, there have been one or two times we've run into sharks who wanted to make a very unequal trade.
So, step back and let DD make the trades *she* wants to, but be there to politely cut off a trade with someone who seems off to you. Now, my daughter has traded rather a lot and I have only stepped in twice, so it's not a regular occurrence at all, just something to be aware of. We've skipped formal pin-trading sessions to avoid it as well.
Your DD is going to love pin trading. There are so many CMs to trade with. Some will simply trade, some will joke around with us for a while. DD still talks about the CM who wanted to trade his pin for her shoes.
Oh, almost forgot...... we didn't have time to do this, but at WDW they have a scheduled pin talk. The CM talks about how pins are made, the 'rules' of trading, and I think you are given a pin for attending. That might be a nice way to get started. Otherwise, just dive in and let her approach CMs. Give her pins you don't mind seeing go and let her choose anything she likes and she'll have fun.
Enjoy!