Maybe the child is sad because of something that happened in the story. And not being white could just be a random design choice.
The cartoon black child could be sad because he didn't get good grades at school or had a fight with a friend. There could be many reasons.
It could be that the child had an argument with a sibling or parent before getting in the car and is still upset. Or perhaps they're just not feeling well and the car ride isn't helping.
It could be that they faced some kind of difficulty or disappointment. Maybe the child didn't get what he wanted, or the mom had a bad day at work and it affected them both.
It could be that the dog got sick or they had an argument with someone before getting in the car, making the child sad.
Maybe the child is sad because they are going to a place they don't want to. Or perhaps they are worried about the dog's well-being.
Maybe Eeyore is sad because he had a bad day or something didn't go his way.
Maybe she's sad because of a loss or a difficult situation. The black and white could add to the mood of sadness.
Maybe it's lost its favorite toy or had a fight with a friend. That could make it sad.
It could be that she's alone and feeling lonely, or perhaps she failed at something important to her.
Maybe it's because of some family problems or a lost job. Those could make him feel down.