My daughter is about 4 feet, 55lbs. I got her size 10. They fit snug on her hips but not her waist. By the end of the day, the underware is a little more slack throughout.  It looks and feels thin so I'm afraid to wash it as her regular underwear.  However, they are holding up well. I think it's more "fear of unknown".  Anyway, I have then and will update if anything significant happens.