I found myself altering books that made references to how boys and girls always hated each other and didn't play together, because when my kid was five he had *no idea* this was an important boy meme. I just didn't want to start putting the idea in his head.
I find myself more likely to hide ideas than words, though. I've even read curse words to him, which surprised me and doesn't always happen if I'm awake enough to think ahead.
I hear you on not wanting to put ideas into their heads. There's a ton of stuff floating around Little House in the Big Woods that doesn't thrill me, but the fact that it was written so long ago and takes place even longer ago makes it easier to deal with. Just about everything about Laura's life is different and I think my son mostly understands when I talk to him about the differences. Last night we read about Laura getting whipped, and he asked me what that meant; my explanation included the idea that most parents today don't believe in whipping children. (If you disagree, please don't tell me about it...)
He asked me last night if Laura was a real person; that was so exciting to me! I started the books just hoping they would be something we could enjoy together, but to see him getting a sense of history from them is a big bonus. I'm afraid they're going to get too sad for him a few book down the road, but we shall see.
(When I told him today was Dr. Seuss' birthday, btw, his comment was "I thought Dr. Seuss was dead." Hard to argue with that...)