The "above the knees" is really pushing it for infants/toddlers. Keep in mind that their proportion is so different from an older child. Even if a dress is "longer", it's hard NOT to have the diaper/undies show.
On a more humorous note, Rachael's friend did her Senior Pictures. In one of them, Rachael was sitting on the ground in a knee-length dress. But the way she was sitting and the camera angle showed right up her dress, and you could see her underwear
!!! Believe me, we deleted that one right away!!! And the young lady taking it hadn't noticed. She's a very sweet young woman who is VERY modest, and would NEVER have sent it on to me had she noticed. I did give her a hard time about it, though