The innocence of the child mind, and their closeness to the after life(we all exist as spirits before we’re even born because the after life exists outside of time) via their closeness to their birth, allows small kids to see ghosts, and other higher dimensional entities that adults might not otherwise see. Most likely Grandma can’t let go of her life, and feels she has unfinished business. Until she willingly passes through the light into the heavenly planes in the dimensions above, she will continue to haunt your family. Although she’s not trying to scare anyone. She just wants to live among you again.
I suggest trying to talk to her about moving on, and focus on the fact that you will all be together there once she crosses over because Heaven is outside of time so everyone who died in the past, present, and future is already there. If that doesn’t work then tell throw caution to the wind, and tell her to actually try and re-enter this reality to live again. She will need to focus her pure thought upon this reality, and enter as pure energy which will cause photons to form with a simultaneous electromagnetic field. If she’s successful she can become a solid hologram in our reality, and be a member of your family again. It’s beyond slim she can achieve this but if she does it will be a weird adjustment for everyone.