Build a rough wooden manger [or paint a small cardboard box to look like a manger]. Set it in some important visible place in your home. Keep some straw nearby. [If you can't find straw, craft stores have raffia or other straw-like materials]. Have a doll or other bigure ready to wrapped on cloth and serve as the baby Jesus on Christmas Eve.
Every time someone in your home does a good deed - an action to help someone else, an unexpected kindness, a small sacrificice, a brave waiting or even a job well done - that person places a piece of straw in the manger. These are the things we do for others that go unnoticed, unheralded.
As Advent continues and the good deeds continue to increase, the manger softens, ready to welcome the baby Jesus.
On Christmas Eve, carry the manger, now soft with straw, to the place of honour where it will spend the night. During the night, place the doll or figure of the baby Jesus in with waiting manger. In the morning your children will see the baby Jesus, lying in a soft bed of straw - a soft bed of kindnesses that they have created.
Encourage your children think of these as secret acts of kindness. They can, at any time, add straw to the bed without having to explain what the kindness was. This helps the child see that the acts of giving or kindness, even if no one recognizes us for doing them or thanks us. God notices them - each good deed we do to one of the least of our brothers or sisters we do to Jesus [adapted from Gertrud Mueller Nelson's To Dance with God]