I think this is from Simple Modern Sewing (Japanese pattern book)
Sewing jeans: Muslin 4
Good post on fitting a muslin - It was especially helpful for me to see the fabric pinned out of the front crotch area.
FREE SEWING PATTERN: T-SHIRT FOR KIDS
Free sewing patterns: free printable pdf patterns and tutorials Get access to hundreds of Free sewing patterns. Each simple PDF printable file comes with the free download and a step by step sewing tutorial.