I got the size 4t for my 38" tall, 32lb, 2 year old.  They fit him great, but they are a little bit long.  They come down to about an inch above his knee.  Not a big deal but I wish they were just a tad shorter so you couldn't see them as much when he wears shorts.  But they are very comfy and have a nice covered waist band that is soft and flexible.  Of all the boxer briefs we tried (multiple hanes style, target, and gap) these are the tightest throughout and the legs don't ride up.