Personally, I’ve never really thought about this before, and I’m not sure the Bible has much to say on it. Just where there is no law, we shouldn’t be making one (we don’t want to be Pharisees…). We just know that whatever we do has to be glorifying to God, which implies loving our neighbor (which includes the infant being photographed).
I think we need to consider three things:
1. What’s the image conveying?
2. How would my model feel about this picture in twenty years?
3. Who’s going to be looking at this image? Just the parents and family in a photo album? Or a larger audience?
For me personally, I have no real problems shooting babies down to the diaper. Maybe it’s because I have a lot of little siblings, and thus have a lot of experience with babies (I’ve probably changed more diapers than most eighteen-year-olds on this forum), but for me, that’s not really a modesty issue. If I see an image of a three-month-old baby in a diaper, my first thought is “aww…. a cute baby.” That’s what it conveys for me. Would my model appreciate that image? I don’t think most people would care.
I would feel a bit squeamish about shooting a baby all the way “as shipped”—that definitely crosses a line for me.
So to convey that to my client I would just be up front in my advertising for infant photography. Maybe on the contract: “please, keep your baby in a diaper.”
Hope this helps a bit.