Pattern for pants in sizes: Preemie, Newborn, 6 Months, and 12 Months.
This pattern is worked top-down, in the round. The waistband is double-knit, and can be finished with elastic or an iCord draw string. There is a gusset there will need to be seamed, preferably with Kitchener stitch. You will then separate and knit the legs, finishing with a 1x1 ribbed cuff.
There are images to help with the steps.