Hi, Wendy. Thanks for another great question for the Disney Parks Moms Panel.
Our daughter was seven when she first rode
Guardians of the Galaxy - Mission: BREAKOUT! She had also been riding The Twilight Zone Tower of Terror attraction since she was only four years old. She liked the Twilight Zone-themed attraction. But when Rocket and friends were captured and compelled to break out, with our help, our daughter was caught up in a whole new level of awesome! Contrast that with our oldest daughter. She had been on the Tower of Terror attraction a couple times. But she had a really hard time with it. Her anxiety about the attraction led to her refusing to join us after that, even at 17 years of age. But, now here's the thing, when the new Guardians of the Galaxy attraction opened, she tried it out with us. And she loved it! When the rest of the family retired to the hotel for the evening, she would stay behind to go on it multiple times by herself! For her, the anxiety over the previous incarnation of the attraction was more to do with the theming than the actual ride mechanics.
Videos compiled by the Disney Parks Blog of experiences of guests as well as descriptions from Imagineers may also help inform how you and your Mouseketeer might approach the Collector's fortress. Since you know your child's interests and fears the best, I'm sure you will make the right decision about whether to invite your little one to join you aboard Tivan's gantry lift. Also, note that most of the important story elements occur within the queue and introductory scene. Guests are welcome to go through the queue and pass through the exit without boarding a gantry lift, if they choose. Just ask a Cast Member.
Have an awesome time, Wendy!
~Tony