You are right; children naturally get excited about Christmas, and sometimes it's hard to get them to see beyond the gifts and the glitter. But I believe we should be thankful for their excitement, because it gives us an opportunity to point them beyond these things to the true meaning of Christmas.
One place to start might be to ask them why we give gifts at Christmas. They may never have thought of this before–but it will give you an opportunity to tell them about the baby Jesus, and how the wise men journeyed hundreds of miles to find Him and bring Him gifts (you can find this in Matthew 2:1-12). You then could ask them why the wise men did this–and the reason was because they realized God had sent His Son into the world.
This could open the door for you to tell them an even greater truth: Jesus was God's gift to us. The wise men gave Jesus gifts that were very valuable and rare; but God gave us the most valuable gift of all, because He gave us His Son. And why did He do this? He did it so we could have our sins forgiven, and could go to be with Him in heaven forever. The Bible says, "For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord" (Romans 6:23).
Let me encourage you and your family to take a few minutes every day between now and Christmas to read about Christ's coming from the Bible, and to thank God for giving us the gift of His Son.