What the heck is "father christmas"?????
Tell your children that Christmas is the holiday when we celebrate the birth of the savior, Jesus Christ. We celebrate it by giving each other a gift to celebrate the day. Christ gave us all a gift by his birth on Earth, its called Salvation, and its called being forgiven of our sins - the greatest gifts of all.
Children love santa claus, of course. First of all, most parents wait until the child is at least old enough where they won't fall to pieces when told that Santa is just for fun. This differs from kid to kid, depending on their maturity level. You are concerned about breaking the little ones hearts, but you seem to be more concerned about your guilt, so much in fact that you ARE willing to break their little hearts to make YOU feel justified. Look, Santa Claus isn't real, so what? When they get older and understand, you tell them he IS REAL. The REAL santa claus is mommy, or dad, or both - hey, isn't that true?? Who brings their presents and puts them under the tree? You do, Santa!!!
But you concern me, because you mention this "father Christmas", but you do not mention Christ. Thats the first part of your word "CHRISTmas", friend. And THATS what its really about. If you want to do right by your children, you'll tell them the truth- the truth is that Christmas is about Jesus Christ. period.