They might not realize that they are actually learning, but in fact, they are. It is very important, though, to teach without telling. Showing will work better, as always. When they introduce themselves for the first time, emphasize their uniqueness. This is the first step to teach human rights in early childhood.
Most children know how to compare and they find it quite amusing.
Show their physical differences and compare them with the other children's. Conclude with an interesting remark that this is natural. Everybody is different and we respect that. In the second step, the teacher might want to bring interesting pictures of people from all backgrounds and all walks of life. Show them how people are different in other places.