I actually learned the danger of telling versus showing when reading, long before I ever started writing. Early on in the book The Golden Compass by Philip Pullman, the reader encounters this paragraph:
“Half a dozen brats turned with expressions of derision, and Lyra threw her cigarette down, recognizing the cue for a fight. Everyone's daemon instantly became warlike: each child was accompanied by fangs, or claws, or bristling fur, and Pantalaimon, contemptuous of the limited imaginations of these gyptian daemons, became a dragon the size of a deer hound.”
Which is very much in line with the Lyra we get to know throughout the book. Incredibly clever, quick-witted, a skilled liar, and highly imaginative--or so I thought.
“It wasn’t Lyra’s way to brood; she was a sanguine and practical child, and besides, she wasn’t imaginative. No one with much imagination would have thought seriously that it was possible to [spoiler redacted]; or, having thought it, an imaginative child would immediately have come up with several ways in which it was impossible. Being a practiced liar doesn’t mean you have a powerful imagination. Many good liars have no imagination at all; it’s that which gives their lies such wide-eyed conviction.”
I recall reading that first sentence several times, getting increasingly angry, and finally setting down the book. I wanted to shake the author and tell him that he was wrong about Lyra! She was imaginative and bright and brave all at once. She could be practical while having a keen imagination, and an imagination can counteract the bald logic that sometimes gets in the way of hope and action. I felt so angry that I didn't pick up the book for several days, and I still cringe when I read that paragraph.
Of course, as an author, I'd like to believe that I understand my characters better than anyone. But statements like that--telling as opposed to showing--can undermine a great deal of the characterization readers have inferred from your books. This is, of course, just my opinion; I've met plenty of people who agreed that Lyra was unimaginative. But I would have liked to make up my own mind about that, rather than having the author tell me what I should be taking away from the story.
I was in elementary school at the time, and I still felt that strongly about it, so I don't think children's or YA fiction should be any different in that regard.
Without a doubt, I am not perfect about avoiding telling. But I try to keep it as much out of my novels as I can. Showing what a character does is much more powerful. It's difficult to bite my tongue when characters make false assumptions, but all real people make those mistakes. It's part of why life can get so messy. I just hope that intent comes through for readers.
(Note: I've been feeling a bit better off and on lately, but I apologize if this entry is a bit incoherent. I probably ought to get a little more sleep, haha.)