Here is a toddler summer dress I made for Misty. I drafted the pattern from a dress she already had. Lexi picked out the fabric for me. I have a pattern for the top half here, and the bottom is just a large rectangle I ruffled. The size of the rectangle will largely depend on your toddler and how much ruffle you make. Since it seems inevitable that my kids grow taller before they grow wider, I added extra fabric at the bottom and hemmed it up so I can let it out as she grows over the summer.
To make the dress simply download the pattern. The top front requires two pieces cut on the fold. The second piece being for the facing. Sew the facing together everywhere except the sides and the top of the straps. Then top stitch the the front and facing to make a nice finished look. Now sew the back facings together and top stitch them as well again avoiding the sides and top straps, but make sure to have both a right and a left side. Once complete you can sew the straps together and the sides. Now you can sew the bottom half on, which again is a giant rectangle. The length of the rectangle would be the desired length plus an two inches for seam allowance and hem, and the width would be the part sewn onto the top and that measurement is dependant on how you ruffle or rouch the fabric. I found it best approach is to just use the extra scrap fabric and trim off the extra.
To finish the back, you can add buttons or a zipper. I added buttons, which added a nice touch to this dress, but a zipper would be just as easy. Which ever you choice make sure you close up the back appropriately. Buttons require a bit of an overlap on the top fabric when sewn onto the bottom.