My suggestion is that you give her a Brother mechanical machine to start off with. They are simpler to learn on and you can get one for around $100 with 27 stitches at Walmart.
The machine is a good beginner machine, but not absolute bottom of the line. I have two of them; one in NJ for when I visit, and the other in my garage/sort of studio so I can sew while the HQ and PC Quilter quilt another quilt. It works just fine, has a true stitch, etc.
It's a bit noisier than then a computerized sewing machine, but that's about it's only short coming. And it's not that much noisier.
If she does happen to break it, or decides that sewing is not as much fun as she thought it would be, then you're not out of pocket for too much money.
Good luck and have fun sewing with your granddaughter!