General mountain dulcimer or music discussions
Lorilee, my oldest grandson is also on the autism spectrum. He is 13 and in junior high school (8th grade). Early on I tried to interest him in dulcimer, but was unsuccessful. He is in the middle school band and plays percussion. His father helps him. His father is not a percussionist, but plays guitar, trumpet, and piano and has picked up some percussion from being in bands. Our grandson seems to identify with the various patterns of drumming as well as different intensities (soft to loud). I don't know if one of his stims (watching things spin) is related to his drumming or not. He is not particularly proficient in math but is very good a remembering numbers.
You could approach your granddaughter by doing some simple counting. Count the scale 0 - 7 (if in DAd) or 3 -10 (DAA). Start with one strum per note, then add a strum to each note, and so forth. You can then introduce different rhythms 3/4, 4/4, etc. I suggest she use her index finger to do the noting even if you have to lower the strings in pitch a little bit. Holding a noter can be frustrating until you master the technique. Slipping in to the middle string causing an extra rattling noise can be disturbing. Of course, you can show her how to hold the noter properly and she how she does. Take it slowly in small increments at first. Don't force it. If she shows an interest, help her to be successful at it.
"The dulcimer sings a sweet song."