Perfect! Not only do I regularly make underwear from t-shirts, but I'd been thinking recently about what I could add to the shop for kids to wear on the outside of their clothes.
I went for a simple one strap backpack bag. The seams are all fancy strong knit stitches to withstand the massive number of stuffed animals and books that are likely to end up crammed in here (and because I like making things that kids can use without fear). The strap and cord are both upcycled and thrifted materials too.