I suspect most families with young children have this problem; I know we did! And after all, don't we want our children to be excited about Christmas, even if it isn't always for the right reasons? As time passes, they'll grow in their understanding.
One way to deal with this is to help your children understand the meaning of Christmas before it actually arrives. Christmas is only a few days away now, but why not take a few minutes daily until then to read to your children about the birth of Jesus? (You'll find it in the Bible in Matthew, chapters one and two, and in Luke, chapter two.) Tell them that one reason we give gifts at Christmas is because on that first Christmas, God gave us the greatest gift of all -- the gift of His Son, our Savior.
Then let me encourage you as a family to give gifts to people who are less fortunate than you. Let your children have a part in this; they might, for example, pick out a toy for a local toy drive, or give some of their allowance to an organization that helps poor children in other parts of the world. It will help them become concerned about others, and not just themselves. The Bible says, "Share with God's people who are in need" (Romans 12:13).
Finally, ask God to help you make the meaning of Christmas a reality in your own lives. Children learn from our example as well as our words; do they see Christ in you?