The Christmas Treasure - The advent calendar book for young and old

By chance Matthias overhears the parents on the evening before Christmas. There is talk of a treasure, a very special treasure. Soon he is supposed to arrive and then there is a big party.
The children, Matthias, Emma, Sara, Lena and Hanna, secretly sneak out of the house at night to search for the precious treasure.
Will they find it? What do old Martin and the farmer's wife Barbara have to do with it?
And who in the world follows the children all the time?