The answer is in your own post. "He understands students, understand Chinese, he knows to select words which kids can understand".

Just because a person has a Degree, that does not automatically make him/her a Teacher. The degree means the person has reached a certain level of education in their own life. It does not mean that they are able to pass on their knowledge to others. Especially not in an interesting and/or relevant form, which can be clearly understood and absorbed by the students.

Nor is everyone cut out to be an instructor/teacher for young people. Personally, I shudder at the idea of teaching children below the age of 10 years old. All of my teaching has been with students who are in the Middle school, Seniors or adults. I think young children are very nice; I just don't want to have to teach them. Other people would prefer them.

If you have a teacher in a situation which is unsuitable and incompatible with his/her psyche, then you will have problems....even at only 18 students to the class.