It seems like I'm always buying souvenir tees at every baseball and hockey tournament we attend. Maybe I'm in sympathy with the vendors since that is my line of work as well. :-) Unlike most kids, my son just never seems to wear them and they languish in a drawer. Since he loves boxers I decided to try to use up some of his t-shirts that way. I used two adult medium tees to make one pair of adult medium boxers, trying to place the printing where it would make the most impact. As you can see in the photo, the tee I used for the front had printing front and back. The one I used for the back (not shown) only has printing on one side because the tee was only printed on the front. If you wanted printing on all four pieces you'll just need more t-shirts or limit yourself to those that have printing front and back.
This was a fun and quick project - less than one hour start to finish. I used Kwik-Sew 2257, modifying the length. This project got a big "thumbs up" from my teenage son (unlike some of my projects) and would be great for girls as well. Happy sewing! Gigi