Maybe he was just really tired from a long day of adventures.
It could be that the boy in the cartoon needed sleep to recharge his energy after a bunch of exciting activities. Or perhaps the story made him sleep to set up something interesting for later.
Maybe he was tired and the hay provided a comfortable place to rest.
The reason could be that the bear had some sort of problem or was under a magic spell that made it sleep through Christmas. Another possibility is that it was simply not interested in Christmas and decided to sleep instead.
Maybe it was part of the plot to create a dramatic moment or teach a lesson.
Maybe they had a fight or a disagreement and the first boy got really angry.
It could be that there was an accident or a villain's action that led to the little boy's death in the cartoon. Sometimes, such events are used to evoke emotions from the viewers.
It could be that he saw something that made him stop. Maybe a barrier or an unexpected event. Also, he might have reached his destination.
Maybe the boy did something naughty and the girl was trying to teach him a lesson.
Maybe the boy did something naughty and deserved it.
Maybe he broke the school rules or was too naughty in class.
It's possible that the snake was just being aggressive or hungry. Or, the boy accidentally got too close to its territory and the snake saw him as a threat.