Never saw the movie, but my teacher read the book to us in 4th grade (she also read one about the kid making the medicine and the grandma grew/shrank/eventually died I think?) and I don't recall it being scary at all.

Actually, I recall thinking it was kind of ridiculous that this teacher read it to us since she was really, really mean and kids called her a witch and a bitch behind her back. I remember I wondered if she was aware and trying to laugh at us by reading the book or if she just really liked Dahl.