|I make my own mini-piping for children's clothes. Ready-made piping is often too thick for a nice appearance. that is in scale with children's clothing.
For the filler, I use Knit-Cro-Sheen Crochet Cotton Thread. For a 5/8" seam, cut 1 1/2" bias strips of your desired fabric.
Wrap the bias strip around the cording and stitch with a medium stitch length as close to the cord as possible. On my Bernina, I use the #3 buttonhole foot and a center needle position. I feed the cord through the groove to the left of center.