What could cause a fever with no other symptoms in my 9 months old and my 3 year old?

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.