It could be that he failed a test at school or lost his favorite toy. There are many possible reasons for his upset expression.
Perhaps he saw something sad or got scolded by his parents. The reasons for his being upset can vary a lot.
Maybe he had a fight with his friend or didn't get what he wanted.
Maybe he had a fight with his friends or didn't get the toy he wanted.
Perhaps they lost a game they really cared about or saw something sad. It's hard to say exactly without more context.
Maybe he heard something he didn't like or was exposed to a loud noise that bothered him.
Often, an upset boy cartoon could be about dealing with emotions, like learning to cope with sadness or disappointment and growing from those experiences. It might also show how the boy gets support from friends or family to feel better.
It might be that the cartoon cat is upset because its home got destroyed or it failed to achieve a goal it had set for itself.
Maybe the kid didn't get what they wanted or had a fight with a friend.
Maybe the kid didn't get the toy he wanted or had a fight with a friend.
Maybe she lost her favorite toy or had a fight with a friend.
First, decide on the poses and expressions of the boy and girl. For the faces, start with ovals and build up the details. Pay attention to the proportions and symmetry. For the clothing, think about the style and folds. Also, add some background to make the drawing more interesting.
Maybe he had a bad day or lost something important to him.