Which is right: It's Christmas Eve...The children are in bed now. Dad is putting the decoration on the Christmas tree.Tomorrow morning when kids wake up, they will be surprised to learn that the tree will have been decorated/is decorated/was decorated/has been decorated by their Dad.
Context, context, context! Any of them may be right, depending on what exactly you mean. Here are just a few examples.
They expect to participate in decorating the tree at Christmas next month; they will be surprised to learn when I call them tomorrow that the tree will already have been decorated by my Dad before they arrive on Christmas Eve.
In their family, the children always decorate the tree; they will be surprised to learn that in our family the tree is decorated every year by my Dad.
In our family the children usually decorate the tree; when they arrive tomorrow for Christmas Eve dinner they will be surprised to learn that the tree was decorated this year by my Dad.
They expect to participate in decorating the tree tomorrow; they will be surprised to learn that the tree has been decorated already by my Dad.