Kwik-Sew’s Sewing For Baby is a great resource! The book has multi-sized patterns and instructions for sewing and serging baby things. Also included are many ideas for adapting or updating the patterns.

I made this sleep sack for my grandson by using the sleeper pattern and extending the bottom to a bag shape rather than fitted legs.
The main body of the sleeper is a cotton flannel and the cuffs and neckband are a cotton lycra jersey.

The zipper is a lightweight flexible nylon. My original plan was to add a zipper across the bottom so diaper changes at night would be easier, but I can’t remember whether or not I included it! Talk about memory problems…

In any case, it looks as though it won’t fit much longer…