I love this pattern I bought from Little Chickadee Patterns Etsy shop. It's so simple and takes hardly any fabric. You could totally use an old t-shirt to make one. This was my first attempt, and I've cut out three more. Next one I cut out, I'll adjust a little to make the front gathered just below the neck, and I'll also use a contrasting fabric for the bias tape neckline and the waistband.
I love rompers on little girls, so summery!