Well I'm scared.
My girl are 10 and 12 and they've each been sewing for a couple of years. They've made sock creatures, mug rugs, doll quilts, lap quilts. However, I cut using the rotary cutter for them and when they cut they use scissors.
I'm 40 y.o., I've been around sharp things (axes, blades, etc) all my life. I'm very careful, very adept at cutting with rotary cutter.
Tuesday night I cut the tip of my finger off. It just took a moment, someone watching me would never have known that my attention wandered for just that second, I broke concentration for a moment to wonder if I had enough fabric left on the section I was cutting (it was a WOF at 4.5 inches that I was cutting into 4.5 inch blocks) to cut one more square.
I lost part of the tip, part of my nail and nailbed and a good piece of the corner of my finger. Unfortunately it is my left hand, index finger. I'm left handed. It'll heal but I'm off work for a week, every thing I do, I do with my left hand, except hold the rotary cutter.
it's ok to not let your granddaughter use the rotary cutter...do the cutting for her, let her use scissors.
My cutter didn't even stop, just kept going and it was all over so quickly, there is no way you would be able to stop someone from cutting themselves.
So, do what you think is best, don't be afraid to err on the side of caution...my girls have no problems using their scissors and they do a lot of sewing.
(ps: my daughter had to blog for me, in case you wanted to read, it's not graphic!!!! just her perspective. www.quiltingunderthemidnightsun.blogspot.com )