Children's books have multiple audiences to please -- not only the kids reading them, but the adults selecting/buying/promoting them. Is it tougher to market a book that kids love but adults groan about, for whatever reason? (potty humor, sexual edginess, etc.)Of course. The children's book market is absolutely lousy with books that are not for children at all (The Gift of Nothing) but that adults will buy for children anyway. Are they easier to sell than Grossology and Everyone Poops, even though those are truly books for how children are, what they're interested in and amused by? The answer is: Duh.
Here again is one of the inescapable contradictions of children's books—the people with the money to buy children's books often have an extremely limited recollection of childhood. I'm not saying for a second that this excuses "children's" publishers for publishing books for the childhood adults think they remember. But it is a fact of the market. It's also something worth fighting, so feel free to grab your banner and follow me.