Honestly, if she has severe sensory issues, you may not be able to get her to wear the earplugs at all. I tried for several years and tried every brand on the market. My DD will wear headphones because she became used to them by wearing them in the car watching her movies. If all else fails, you can try that route.
During her first trip to Disney, we found the parades way too loud for her liking. The best we could do for the noise was earmuffs, a hat, and our hands over her ears. It was a real shame because she really liked Spectro otherwise but she couldnt tolerate the entire parade. We have just tried to be creative with her. Last year, we made ADRs at Tonys for dinner and watched the parade from the restaurant. We watched the fireworks from the resorts instead of the parks. Not an ideal solution, but it worked best for her.
My DDs sensory issues are better some days than others and it would show when on some of the rides. For example, some days, she loves Pirates and other times, she would cover her ears the entire ride. Same for Small World and Pooh. But we wouldnt have known if we hadnt kept trying. Also, we found if we gave her some calming sensory input in the room prior to leaving in the mornings, she did a little better. That may or may not work for you. If your DGD has a good OT, I would ask him/her for suggestions.