I am looking for a cartoon where a young boy has a flying bed of sorts and he's on it and is flying to a castle or a palace in the sky. When he gets to the castle there is a shadow monster or some type of shadow creature. I don't remember much about it.

I'm not sure but the only one which comes to mind is, Little Nemo: Adventures in Slumberland. (http://en.wikipedia.org/wiki/Little_Nemo:_Adventures_in_Slumberland)