The art of improvisation is a good place to start to introduce music theory. It can be much more fun than sitting in front of a blank sheet of manuscript paper. Improvisation is not only useful in teaching harmony, it's also a way to teach form. If the student is having trouble finding motivation to improvise, suggest either a specific mood, story or scene. Watch a short film with music, and have the student identify the type of music played to illustrate the mood. Ask the student where the climax occurs in the film and music.
Many children will have trouble understanding the concept of primary chords and why they are so important in western music. Before teaching a child the function of a certain chord have the child play various types of triads (three-note chords), not just major and minor triads but diminished and augmented chords as well. Ask the child what type of mood each chord has.
Explaining rhythmic units to children involves some creativity. If you try to explain how the beats are divided in an academic manner then the child will likely loose interest. A fun game to teach children how to count rhythms is to use a corresponding word with the same rhythm. For example, an eighth note followed by two quarter notes would normally be counted one-and-a, instead use the word "grass-hop-per" which has the same rhythm. Have the student come up with her own words for each corresponding rhythm.
Teaching aspects of musical melody can be abstract and confusing unless it's done creatively. In their book "Integrating Music into the Elementary Classroom," William M. Anderson and Joy E. Lawrence devised a game designed to teach children aspects of phrasing and melody. They suggest using a simple song with many repeated notes such as "Micheal Finnagan." The teacher shows the song sheet with different colors for each musical phrase. Each color corresponds to a sentence of the song. Next the music teacher asks the students what is different or the same about each musical phrase or sentence.