|This is gathered from various sources. I sew a lot of knits in the winter: PJs, sweatshirts, etc. I cannot always find "ribbing knit" in the color I want, but can always find a suitable color of a rib knit (usually cheaper anyway). The fastest way to do a neck: Sew one shoulder, sew the entire neck rib on, then sew the other shoulder and neck rib in one. The seam will be at the shoulder, not the back. Do the same thing for the waistline rib: cut one long piece (not two separate ones if the pattern calls for it), sew one side, then sew the waist rib along the entire bottom, and sew the other side, rib included in one. For sleeves, always do the cuff or rib before the side seam: cut one long piece of rib, plus about 1/2", instead of one piece for each sleeve. Then attach the rib across both sleeves, and cut apart after you're done. Starting the rib is the worst part, and this saves you one of the starts. I mark the mid point: so I know when to begin the next sleeve, or where the side seam should go (on the waistband). Using this method you can complete a shirt in under an hour easily.
Patterns often have you make a circle of your ribbing and add it to the sleeve/pants/waist. On children's clothing, especially, this gets you into very tight situations fraught with peril! This is much simpler, faster, neater.