i do not know how i would treat children if i had them.
i would never tell them any lies, but there is a bit of a dilemma when it comes to the fairy tales that most children believe.
if i said santa was not real, then they may be disappointed or they may develop a cynical attitude toward others who propound the idea, and they may be seen as sullen or distant by their classmates.
if i said he was real, then at some point they would rationalize that he wasn't and maybe consider me to be either a liar or stupid, and that may compromise their belief in further advice i may have to dispense as they grow older.
i think i would pretend that i am not sure if he exists or doesn't. i would place myself at their level of curiosity and wonder with respect to santa.
that would seem to me to be a good compromise between lying and shattering their belief in simple things that would help them fit in with their classmates and contemporaries.