In this case it's likely partly a signal to noise problem that can't be mitigated easily. Both children and dark skinned people produce less signal to a camera because they reflect less light. children because they're smaller, and dark skinned people because their skin tones are darker. This will cause issues in the stereo vision algorithms that are finding objects and getting distance to them. Lidar would solve the issue, but companies don't want to use it because lidars with a fast enough update rate and high enough resolution for safe highway driving are prohibitively expensive for a passenger vehicle (60k+ for just the sensor)

Darker toned people are harder to detect because they reflect less light. The tiny cheap sensors on cameras do not have enough aperture for lower light detections. It's not training that's the problem it's hardware.