Quite some time ago, I had a student with a learning disability. She learned beautiful technique, good rhythm, and could play each hand separately perfectly, but she could not play two lines at once. We tried all sorts of approaches for about two years, I used various excellent books on pedagogy, but finally she gave up. I hope she tried a single-line instrument afterwards, because she was musical. I hope you have a wonderful time in this endeavour. You will probably find that each student will be able to play only certain types of repertoire, depending on their particular abilities.