Only kids say "kiddie".

It [the quote] is lovely. I used it for my high-school yearbook quote.

It also doesn’t apply to the MCU, My Little Pony, or whatever else WeirdRaptor and his pals have co-opted it for.

Lewis is specifically criticizing those who criticize a property for not being “adult” enough. That is to say, he is not taking issue with those who consider a work “kiddie,” the word you used; he is taking issue with those who don’t find it adult enough. There’s a subtle but important distinction here, because he’s not saying that one ought to regress to one’s childhood; one has to keep on and move on from childhood. But one also can’t be ashamed of that which is magical and good about the world—that which makes one child-like rather than childish. Lewis was, of course, a Christian, and his defense of childhood is very Christian: “Suffer little children to come unto me, and forbid them not: for of such is the kingdom of God,” and all that. Not the childish adults, but the children, and those adults who have a child’s faith.

Lewis implicitly criticizes childishness when he writes, “When I became a man I put away childish things”—paraphrasing St. Paul—and then comes his stinger, “…including the fear of childishness and the desire to be very grown up.” The fear of childishness is childish and should be criticized on that ground. Well, what does he mean? He means what Christ meant—those with the faith of a child and all that.

So, his point is not to be used to justify a property one likes as an adult. The point is to change one’s whole thing as to rediscover a child’s sense of faith and wonder. Then can one start liking fairy-tales again, which is the point of the essay.

Hope that helps.