Just because you have information about something, does not mean you are responsible for disseminating it without thinking of the consequences. This substitute teacher found out the hard way recently after being fired from her job after disclosing the "truth" about Santa Claus.
"Santa Claus & Easter Bunny" issues are sensitive subjects especially in the lower grades. I had to handle a similar situation a couple years ago in a 2nd grade classroom.
Even if the kids are insistent to know the truth, it should come from a parent. Even then, the youngster might not be quite ready.