We just splurged for my daughter's birthday and bought her some Hanna Anderrson hipster fit underwear, size XS. I think they are awesome, and they are organic cotton. These fit my 27 pound, 2 year old daughter great. I tried them on my 5 mo, 16-17 pound daughter. They were a little big in waist, but they would work fine for her too.
We found that the fit in 2T/3T panties varied widely, but I really like the fit of some TKG ones made in Canada that I found at Sears. We bought lots of panties when my daughter was about 1 year old, and many of the 2T/3T panties fit her well enough then. Look at the weight ranges because some of them are specific down to 22 pounds or so on the 2T/3T size. I thought I had way too many panties, but we use all of them.
Again, I love the Hanna Anderrson undies that we just bought. So if you can justify a splurge for those, I would go for it. My daughter loves them too, and I love the lower rise fit that they have and the incredibly soft cotton. I wish I had bought them earlier!
Keila - whose daughters are well-endowed in the hip and thigh area, and of course, that does affect fit