So I have been searching and searching for a good baby underwear pattern. I have tried a few, and tried making one (disaster! way too small and the legs didn't match up). I also tried modifying a trainer pattern to make it without the soaker. It ended up way too big, with the crotch too wide and the waist too big. I guess the thickness of the absorbant soaker makes a difference in how the outer material stretches.
I happened to be on Etsy and found one for sale. After contemplating for a few days, I decided to go for it ( http://www.etsy.com/view_listing.php?listing_id=27394881&ref=sr_list_1&&ga_search_query=children%27s+undies+pattern&ga_search_type=all&ga_page=&order=date_desc&includes=tags&includes=title). It is sized 2-4-6-8 and there is a pattern for girls' underwear and another pattern for boys' underwear. They are equally cute.
Here are photos of the 5 pairs I have made so far using this pattern. They are perhaps the cutest things I maybe have ever seen.
I was surprised that the size 2 fits pretty perfectly, and I had to modify the leg and waist band length up quite a bit from what she recommends (to 11" and 16", if any of you buy it and want to use it for your 23 pound 11-month old). My next modification is going to be shortening the whole thing at the waist by about an inch, because it sits at the waist and I think it would be cuter below the belly.