I have worked in RTW production sewing. Sew everything you can. Then go to the iron and iron all the seams that you have sewn. Using a toddler's pants for exampe; I'd sew the crotch seams first. Then the side seams. Go to the iron and iron all seams open. At this point, I'd also iron the hem fold and the waist casing fold. Finish seam edges next. Next, sew the inseam, starting at one pants hem and ending at the other pants hem. Iron that seam open. Finish that seam edge. Finish the hem edge and sew it. Now do the waist. You have options there. You can finish the raw edge of the waist casing, fold and top stitch the casing. Then insert the elastic. This method allows you to adjust the elastic to fit the child.
Another method which does not allow you to adjust the elastic is to sew the elastic into a circle. Then either sew the elastic to the casing on the serger, stretching the elastic to fit. This allows you to sew in the elastic and finish the edge of the waist at the same time. Then fold over the casing and stitch through both layers of fabric and the elastic, again stretching as you sew.
Chris--extended breastfeeding, cloth diapering, babywearing, co-sleeping, APing, CLW, homeschooling before any of this was a trend mom to Joy (1/78), Erica (8/80), Angela (9/84), Dylan (2/98)