In a shop, it’s a no-brainer. A customer would walk in, and if they don’t immediately see what they are looking for, they will ask. The sales assistant would likely be able to walk them to where they need to look, the customer may notice a couple of additional purchases they want to make on the way, the sales assistant might ask the size and find it for them, and would likely suggest that they have a pair of pants which have been popularly matched with this shirt. We’re sure we’re not the only people who have left a store with more than we intended to purchase based on products that the sales person has recommended.
When shopping online, it’s unlikely that the red sports shirt the customer is looking for would be directly on the first page. It is logical that they might go to the menu bar and select “Children’s Clothing” and then “Tops” from the dropdown to start their search. While this is clearly what they need, in offering “Tops” as a secondary category, you have already reduced your chances of an add-on sale as the customer hasn’t been exposed to the range of pants that you also have for sale.