Building Block Muslin

Hello! I'm starting with the building block dress and I am making the basic dress. I started making a muslin and I noticed that the pictures of the princess seam bodice has a waist marked. Is this on the pattern? How do I find the waist on the flat pieces? Separately but related: is the waist on a toddler about an 1" above the belly button? Sorry if these are silly but I've only ever made the size on the envelopes and they generally fit ok. Excited to get a better fit and understand how to alter a 2D figure to fit a 3D one. Thanks, Martha