Have they seen any of the Star Wars movies? It’s not essential to have a good time on the ride, but I found that it helped my DD enjoy it more because she knew the characters and some of the planets and could follow the storyline.
My DD has been riding Star Tours since before she was 3 (she’s quite tall for her age). On the same trip to Disneyland, she rode Star Tours multiple times, but after one trip on Pinocchio, she was DONE with that ride because it was “too scawy!” It’s hard to know how they’ll react to rides in advance.
Hard to do if DHS is your first park, but if you are going to other parks first, you can gage their reaction on other rides. Are they fine on the Barnstormer? BTMRR? Test Track? Splash Mountain? That will give you an idea of how well they might handle the ride motion.