If I may just add to the point, to answer your question directly, there isn't a tracking system under the HST that requires merchants to identify how much was sold in children's clothing that qualifies for exemption. They simply send in their total HST collections. Through the input and output tables Stats Canada collects--if they have a category for children's clothing--they can identify the amount sold among the HST provinces. The allocation formula simply takes that into account and slices off a portion, for example, for Ontario in relation to that. It's not merchant-based or individually tracked. It's done overall with the aggregate data.
