I bought these for my 3-year-old daughter.  She normally wears size 4-5T.  I bought these in a size 6 and thought they'd be too big but they fit her fine.  They're tagless and soft cotton, so very comfortable, and have big legholes so they're easy to get on and off.  She's had other underwear that had elastic digging into her around the waist but not these.  Would buy again.