I recently married my childhood sweetheart.
That is awesome, did this come out of about by an extension of friendship or physical attraction?
I would say by an extension of friendship. Do most teenage relationships last?
Not really, because it is a transitional stage of physical and psychological development 
I see, so what percentage of high school sweethearts end in divorce?
That is difficult to ascertain due to the nature of such marriages and the dissolution the bonds of matrimony are more variable at that age.
How do childhood sweethearts show affection for each other?
Usually in a more platonic way as they last only a short to medium period of time. 
Do childhood relationships that end ever get rekindled later in life?
It does not happen very often as such relationships mainly serve as experience for subsequent future relationships.