Many things. Extensive physical exercise will generate elevation of body temperatures, and any form of inflammation, whether infectious or not.
Depends... Despite understanding your concern of unexplained fever in your children, this actually is all but impossible to answer without a physical examination and perhaps even labwork. This a medical quandary that most pediatricians face daily, hence your pediatrician would be the best resource to reckon with this dilemma. There is an explanation for the fever--but w/o further input, it is but a guess.
Periodic fevers. There are conditions where a fever may occur on a regular basis time wise-called periodic fever syndromes. These are complicated and you should discuss these with the pediatrician.