For the tail, fold the quilted fabric in half. Place leggings that fit your child on top of the quilted fabric. Draw the entire mermaid's tail shape on the fabric, referring to our pattern. Cut out two back pieces. With a blue fabric side facing a lime side, stitch the backs together, leaving the top open. Turn the tail right side out. Stuff the fin area with fiberfill and sew across the top of the fin area.
To make the front of the tail, fold a piece of quilted fabric in half. Place leggings on top of the fabric and draw the pattern's blue shape onto the quilted fabric; cut out the front piece. (The outer edges of the front piece pattern should be three inches larger than the leggings at their widest part.) Stack the front tail piece over the back piece and blind-stitch them together along the sides. Stay-stitch around the waistline.
For the waistband, cut an 8x24-inch strip of ruched fabric. With right sides together, stitch one edge of the waistband around the top of the tail. Fold the waistband in half with wrong sides facing and baste in place with elastic in between. Secure the elastic ends. Slip-stitch the waistband closed.
For the bikini top, download the pattern and enlarge as needed. Cut two shapes from the quilted fabric. Sew shapes together with the lime sides facing, leaving openings between the dots on the pattern. Turn right side out through largest opening; sew opening closed.
Cut iridescent ribbon into two 13-inch lengths. Slip an end of each ribbon into the top openings for straps. Sew each opening closed and top with a silk flower and a white pearl bead.
Cut the sheer white ribbon into two 24-inch lengths. Slip one end of each ribbon into the side openings of the bikini top and sew closed. Sew a silk flower and the teal pearl bead to the center of the bikini top.
For the crown, use half of the blue yarn to make continuous 1 foot to 3 feet long loops. Hot-glue the top of each loop across the satin headband. Repeat using half the lime yarn. Cut the remaining blue and lime yarn into 2 feet to 4 feet pieces. Fold each cut yarn length in half and thread it onto a darning needle. Thread a silk flower petal and an iridescent bead onto the ends. Remove the needle, and then secure the petal and bead with an overhand knot. Hot-glue the yarn folds across the headband. Glue sea shells along the headband with the E6000 adhesive.
Suggestion: Wear this costume with a flesh-color shirt and green or blue leggings.