Thank yo for contacting us.
You can try to select parent product (T-Shirt) and child products directly in the grid and export them to eBay via right mouse click>Addons>export to Ebay. Also you can assign child products with parent to one category and export to eBay. Tomorrow our developers will check if it is possible to create custom SQL, which will select child products automatically and export to eBay.
Let us know if any other questions occur!