I have my little speech ready for my boys, and it's what my mum told me when I worked out he didn't exist. She said "Ok, Father Christmas doesn't exist as a person, but the Christmas spirit does, and THAT is what exists in everyone at exactly the same time each year. If you believe in THAT, then Christmas will always be special."
She doesn't remember saying it, but I do, and it made perfect sense to me! (I was about 8, I think.)
Also, if you want to help your kids keep on believing for longer - rent The Polar Express DVD. We all love it here, and it makes Santa seem possible all over again after they've stopped believing. (Just ignore the fact that it's a Tom Hanks film!!!)