Welcome to planDisney, Lauren!
You and I are in the exact same situation. My littlest one is only 2 so there are many attractions that she will not be able to ride. Her older brother is tall enough for almost everything in the parks, and a huge Star Wars fan, so I am sure that I will be purchasing an a la carte
Lightning Lane reservation for this experience.
Lauren, while we do not have any "official" word on how Lightning Lane purchases will interact with the
Rider Switch Service, I am willing to bet that it will work similarly to the old FastPass+ system. This would make sure that everyone got to ride, and no one had to ride by themselves. My son may miss the days of his little sister being too short to ride once he realizes that his Rider Switch days are over.
Because
Star Wars: Rise of the Resistance is such a long-lasting immersive ride, I recommend that you take your little one over to see For the First Time in Forever: A Frozen Sing-Along Celebration while you wait on their older sibling. Also nearby is
Vacation Fun - An Original Animated Short with Mickey & Minnie or
Muppet*Vision 3D, both of which usually do not have a long line and offers some much-needed air conditioning.
May the force be with you, Lauren!
Sarah B.