Other mistake: When Jack first arrives in Christmas Land he doesn't know what is falling from the sky (snow) but he somehow knows that the children are throwing snowballs.
Other mistake: When Jack is handing out presents and the first house he goes to a kid catches him, and he hands him a present. Well you can see the bag of presents right next to the kid, but he never takes it back up, yet has it again later in the movie.
Answer: As seen in the movie, the Fog Juice made it difficult to see anything. Sally had hoped that with no way of seeing where he was going, she would be able to stop him from leaving Halloween Town and save his life.