The fight is underway and just by writing this, you are part of it. Keep up the good work and Merry Christmas.
My kids were raised on the fact that Jesus is real and the reason we celebrate and Santa is a symbol of the spirit of giving. They are planning to raise their children the same way. My kids had all the thrill of anticipating what they would get without having to learn about the lie later.
Without being told to they even talked about Santa to their friends and cousins as if he was real. Even when they were only 3 years old they both told their versions of the Birth of Christ whenever Santa was discussed. In a way I raised missionaries. Both my daughter and son are adult Christian Conservatives now.
My children are young yet, but we have always emphasized to them that Christmas is about Jesus Christ, first and foremost. Like your children, they too will quickly remind anyone in their earshot who seems to have gone far afield, what Christmas is all about. They know "Jesus is the reason for the Season", and each year we do Advent candles, and Advent calendars with manger scenes on them, and we always have a birthday cake for Jesus on Christmas Eve, and we sing Happy Birthday to Him before we cut the cake. I hope they grow up with as deep a faith as your children have! Good job!