Off the top of my head, judging the age of your granddughter, seems to me a bit of sayin it was seen as acceptable at the time but isn't any longer: coupled by a general description of the slave triangle Africa/America/UK would probably cover most. Some indication on how the rich in the UK owned/kept slaves. And mention of the cotton fields in the Southern American States would surely be enough ? Going into more detail is probably for an older child. If she reads a bit about it (local library maybe) I'm sure she could rewrite what has been learnt in her own words.