My 2.5-yo girl is 23lbs wet and wants to wear big girl underwear. In my research for really tiny underwear for toddlers I've seen that people recommend the Hanna Andersson brand. I picked them up in the smallest size and they're still too small, despite my attempts to shrink them. What else have you tried that worked? She's super skinny, still wearing 12 months shorts.
Yet Another Small Underwear for Small Toddlers Thread
Do you have an H&M where you live? They have size 1-2 underwear that are pretty small. We did EC with my tall, skinny, boy and I think they started fitting him around 18 mos or so. Also, they are WAAYY less than the Hanna Anderson (which are lovely, don't get me wrong!) and you can get them without licensed characters if that's important to you.
We use Hanes size 2t/3t. My daughter is 23 lbs and they fit fine. We get ours at Target: http://www.target.com/p/hanes-toddler-girls-6-pack-brief-assorted-colors/-/A-14042154