I have a daughter, Cassidy, who has special needs. She is nearly nine. Having watched her sisters enjoy pin trading over the last few years she wanted to take part in the fun during our most recent vacation. Unfortunately the fine motor skills that are required to manipulate the pins are a challenge for her and she isn't quite able to do so on her own. The Cast Members we encountered were all incredibly patient and understanding when it came to trading with her.
When my daughter spotted a pin she wanted to trade for, we taught her to approach the Cast Member and politely (with emphasis on politely; we're working hard on that!) ask to trade. Once the Cast Member accepted her invitation (which they always do) Cassidy would point to the pin she wanted. Then, the Cast Member would ask which pin Cassidy wanted to trade and she'd pick one from her own lanyard. I'd help her to remove it and replace the pin back so that nobody accidentally got poked. I'd give the pin to Cassidy who would then trade with the Cast Member. It was as simple as that.
You can read all about official etiquette of pin trading
here. If special circumstances prevent you from following procedure to the tee, don't worry; Disney Cast Members are among the most helpful folks you'll meet and they are more than happy to pin trade with little ones who are just learning the ins and outs of the hobby!