Lewd humor, well if you want to risk doing that I'm not going to say no. After all, there is a proud tradition getting shit past the radar in children's stuff. Even in today's Disney movies they're hiding dicks in every scene.
It'll be a case by case basis, but sure if you think you're subtle enough then you can go for it. Obviously if you mess up, you'll just fail and that will be the end of it.
Violence, of course. Death even. Like I said, just don't go into major graphic description.
You can say the witch cooks little children and eats them, but you can't say she skins them alive, uses a sharp iron rod to skewer them through their ass and out their mouth like a pig on a spit and feasts on their tender bits first because she likes those the best.
If anyone needs a good example of a guideline for a children's story here:
http://chooseyourstory.com/story/imagination