1. I think both approaches are good for the class, but the one I tend to use much more often is the product approach. I have ESO students and most of them still need a model to follow when producing a written text and the product-approach is better for this. They need to know the basic layout for different types of writing, since they are asked later to know them for their exams. Again, the product-approach makes it much easier.
2. I guess with children the process-approach is more natural and richer. With those groups who are aimed at taking official exams ( Cambridge, etc.) the product approach is better, since the students are provided with the layout, the accuracy and the structures they will be required in the exams and they will be graded.