Everything I have thought of has been mentioned except one -
Make sure they have as many "flex points" as possible and the highest decibel rating as possible.
My favorite for me and kids have a hinge joint on the back of the ear piece, so the ear piece will always fit flat against the head. I often am able to use adult size on five year olds, because they fit flat. However, the jr. size is better, because the are lighter, and the head strap fits correctly.
I also like the 35db rating. With this level of muffling the "fear factor" is almost eliminated. Why let sound bother someone that is getting used to recoil. I have a horrible flinch, and sound is part of it. I developed it when I was young, and I can't completely get rid of it.
Thanks for thinking of what is best for the grandkids!
Good luck.