Hi
I would suggest that your granddaughter's parents ask for help from a Health Visitor. She needs to be weighed and measured to check that she is growing ok. I imagine she's had a bad experience with food (possibly choking?) but whatever the casue she and her parents need some advice and support.
When my older son was about 18 months he went through a phase of only eating bananas, yoghurt and weetabix. I went along with it for a few weeks figuring he'd grow out of it, and then called my Health Visitor who was very supportive. I've also asked for help with other (non food) issues with the kids and have always found them helpful.
This problem needs to get sorted. If your son/daughter doesn't take her to the GP or the Health Visitor to seek help and then your grand-daughter becomes ill as a result of her not eating, then things might look quite bad for them.
I hope it gets sorted out soon, must be very worrying for you all.