Kate from See Kate Sew shares a free pattern for a simple baby top or sundress. It has a basic bodice with ruched straps and a gathered skirt (make it long for a dress, shorter for a top). Her pattern will make a dress to fit a baby 3-6+ months. If you need a smaller or larger size, the sizing should be fairly easy to adjust. Go to her pattern and tutorial.
[photo from See Kate Sew]