Hi Rachael,
Thank you for visiting the Disney Parks Moms Panel for assistance with your great question! My mom has some mobility challenges and I, too, have shared similar concerns regarding the conveyor loading platforms. There are several in particular which come to mind:
Spaceship Earth in Epcot has one that you step onto just as you board your "time machine". Epcot's
The Seas with Nemo & Friends has a moving belt as you board your clam shell, as does the
Voyage of the Little Mermaid in the Magic Kingdom. For some reason, that one seems to move particularly fast, and my mom definitely felt rushed. The "Doom Buggies" on the
Haunted Mansion in the Magic Kingdom are also boarded via a moving conveyor platform. There are other rides as well. The great news is that on each of these aforementioned rides, there is a Cast Member who is standing right at the start of the conveyor. I recommend that you pause prior to stepping on, get their attention, and ask them to slow it down for your family member. They can push a button and slow the conveyor down significantly. I have utilized this option numerous times. Please don't hesitate to do the same.
Wishing you and your family countless magical memories,
Kaylene