Do you tell the kids of divorce 10-9-7 yrs old , not to call the stepmom - mommy ?

is that fair to the kids ? also the father with the new wife to be , is it fair for him to move her in before they are married and insist they call her mom ?? they know she is not their mom . their mom is the one they visit evrey other weekend . they do not want her , they want their mom .