I have found over years of clothes sewing for little girls, the best gathering is done by running two or three rows of stitches. If the area is very long, break it up at a seamline otherwise run one line at 6/8, another 1/4 " in and then another 1/4" in. Pull on bobbin thread, not top line. You can also sew over crochet thread or dental floss with a zigzag stitch. you do have to go back and take out the gathering lines outside your seam line but it is worth it for how nice it looks.