In our family it was done like this: The story of how the presents get magically to the house was told, just like you would tell a fairytale, in this kind of storytelling way. Younger children believe it, older children begin suspecting something from the tone of voice. We also let some things slip sometimes, like hiding presents and having to go and buy some secret stuff to help with preparing the Christmas. Children of older preschool age really enjoy being able to find out themselves, suspecting you and catching the clues. Then when they confront you with their theory, you can let them in on the conspiration by just a wink, maybe tell them not to let others know. They then tend to start participating, preparing their own presents for others. It works very well.