My 9 yr old LOVES pin trading.
I agree with the previous poster. Make sure that you supervise the trading, but I'd say spend some time getting familiar with what you'll trade and what you may see, in terms of edition sizes, value, cost, etc. Only because I have found many surprises in the lanyards and trading books and you would not want to miss out on grabbing a good pin that someone else left as a tradable or end up giving a good one away yourself! Experienced traders know about this possibility and will lurk waiting to see who can be taken advantage of.
Another word of advice: Stick to trading with CMs only, at least until you feel more comfortable to trade with collectors, for the reasons stated above. Also, buy pins that you know your child does not like too much, to motivate her to exchange them in trades.
Pins are running very expensive now, even over $10.00 for limited editions. You can go to cheappins.com to buy several tradables for under $5.00. Read the collectors boards here, in Dizpins.com and take a peek at the pins in pinpics.com, where you can get a very good idea of how rare/limited/desirable what you'd trade is. Good luck.
Wanda