With my old 35f2, the lens is OK at F2 and super at F2.2. I would only stop down more for depth of field.
If some pictures are not sharp enough at F2, it is because of motion. I did some tests with bounce flash to remove my hand motion and subject motion from the equation and the lens is fine at F2.2, and good at F2.
I don't know how much of typical blur comes from my failure to hold steady, and how much comes from subject motion in low light. I do know that even with the lens at F2, motion blur matters, along with the effect of body motion changing the effective point of focus.
Higher shutter speeds matter when photographing kids!
Of course, stopping down further helps to sharpen up the corners, but this isn't an issue in low light portraits on a crop body.