Parents have been charged with manufacturing child porn before when they took pictures of their kids bathing to get developed somewhere and the guy working there saw the pictures.
And that would be wrong.
Plus, cases of parents being arrested for taking pics of their kids in the bath really aren't as common as the media would have you believe. Almost every parent has at least one picture of their child(ren) bathing -- many of whom are now posting those images on sites like Facebook -- and very few have been arrested or hassled over it. Hell, my baby album has tons of images of me in all my naked glory, including a couple where my crotch is at least partially visible.
Side note: It would be whack if someone came along and charged me with possession of child porn over my own baby pictures.
Which I have to wonder how you prove that a character in a drawing is underage and doesn't just look young like the porn star gauge. I've also heard them say it's not that the child is naked, the child could be in what could loosely be call a provocative pose(unintentionally). It's the person's intent that matters. How the fuck do you prove what someone was thinking? Thought police?
Thing is, almost anything can be viewed as pornographic if you look at it the right (or wrong) way. It's also very difficult to legally define what is and isn't pornography. I mean, there's the whole "intent to elicit arousal" line, but like you said, it's quite hard to prove someone's intentions when we get into that weird, grey area between pornography and non-erotic nude photography.
To be honest, I can understand why some people might worry about the wrong kinds of people getting a hold of those pictures and trading/selling them for the purpose of arousal, thus violating the child's right to privacy, but in those cases, it's the sick fucks who steal and misuse the photos who should be prosecuted, not the parents who innocently took a picture of their baby's first bath.