This is an excellent illustration of the Affinity Bias which is a cognitive bias which is an unconscious tendency to get along with people who are similar to us in various ways (gender, religions, ethnicity, etc.):
The key here is that this is an unconscious bias. The children in the video are not intending to be racist, but they have been raised in a culture that communicates racist assumptions to them which they have internalized. The experiment conducted by the psychologist reveals this. Because these biases are unconscious, the only way to combat them is to draw attention to them so that we can become aware of these tendencies in our thinking.