I often told my students: Tempo is music's heartbeat; it can go fast or it can go slow, but it cannot be suddenly fast and suddenly slow.  Music can get heart disease, too.

Our sense of hearing starts inside the womb.  Mother's heartbeat is our first experience of tempo.  Tempo is supposed to be a habitual repetition of a beat, but some people really mess it up.  When I find it hard to explain tempo to young children who have no sense of rhythm, I would play a Volkswagen car commercial for them to "feel" the tempo. (http://www.tudou.com/programs/view/scsOSSfSQeI/) I remind them that tempo is everywhere.

Some boys are not without a sense of rhythm, but they are simply too impatient and  too smart to grasp it.  I would ask them to close their eyes and imagine themselves dribbling basketball to the beat of the metronome.  After a while, when they put their fingers on the keys, right away, the music began to breathe smoothly.

Many little ones are so busy playing the piano, i.e. reading the music, looking at the keys, and counting the beats, that they fail to hear the music they are playing.  For these students, I would record their practice and play it back for them to listen.  Hearing their own music and their own mistakes is more effective than telling them ten thousand times what mistakes they made.
 
I often remind the kids:Tempo is a feeling; it's a way for the composer and for you to express yourself musically.  I certainly do not want  you to be a mere metronome.  Even in formal concertos, there is a cadenza for the pianist's free expression.  You shouldn't panic because you're totally on your own, and you shouldn't lose this opportunity to go solo and shine by playing the same old things.

Life has many pieces of cadenza.  Have you found your own tempo?