C.S. Lewis often received letters from the young readers of The Chronicles of Narnia, and he was very good about answering them. He replied to a girl who had written to him objecting to Lewis's use of the word "kids":
Although your letter was written a month ago I only got it to-day, for I have been away in Donegal (which is glorious). Thanks very much: it is so interesting to hear exactly what people do like and don't like, which is just what grown-up readers never really tell.
Now about kids. I also hate the word. But if you mean the place in Prince Caspian chap. 8, the point is that Edmund hated it too. He was using the rottenest word just because it was the rottenest word, running himself down as much as possible, because he was making a fool of the Dwarf--as you might say "of course I can only strum" when you really knew you could play the piano quite as well as the other person. But if I have used kids anywhere else (I hope I haven't) then I'm sorry: you are quite right in objecting to it. And you are also right about the party turned to stone in the woods. I thought people would take it for granted that Aslan would put it all right. But I see now I should have said so.
By the way, do you think the Dark Island is too frightening for small children? Did it give your brother the horrors? I was nervous about that, but I left it in because I thought one can never be sure what will or will not frighten people.
There are to be 7 Narnian stories altogether. I am sorry they are so dear [expensive]: it is the publisher, not me, who fixes the price. Here is the new one [The Silver Chair].
~C.S. Lewis, Letters to Children, Letter of Sept 14, 1953
He then writes her again, five days later:
I feel as one does when after "showing up" one's work one realises one has made the very same mistake one got into a row for last week! I mean, after sending off the book to you, I read it myself and found "kids" again twice. I really will take care not to do it again.
~C.S. Lewis, Letters to Children, Letter of Sept 19, 1953