Enid Blyton was a good story writer and children will find her whether they are put in front of them or not. DGD, aged 6, has discovered the Secret Seven and loves them.
I do think with books for younger children should have individual words changed where they are now deeply deeply offensive, like n****r. We would not want them reading books saying f**k or c**t p**i and the n****r word is as, if not more, offensive and there is always the danger that they would use the words in conversation and get themselves unknowingly deeply into trouble.
Older children and adults are capable of understanding that they represent the language and attitudes of a previous age that not now current. Bear in mind we use words and have attitudes now that will deeply shock people of 2113, and they are probably not the ones we would think of.