Why is the thong scene legal?

Can someone, hopefully a lawyer, explain to me how the thong scene could be legal?

It sure seems like it should count as child pornography. Heigl was only 14 when this was filmed and the thong completely exposes her butt. And I don't mean "virtually" or "almost." I mean completely. The center strap is completely hidden in her butt crack and the cross strap is way up in the small of her back, so every square inch of her bum is visible including the crack.

This is NOT comparable to a bikini, even a thong bikini. In fact, what she's wearing is not actually a swimsuit at all. It is a thong leotard and it was popular in the late 80s and early 90s, but it was one layer of a two layered workout outfit. Sometimes gym shorts were worn over it. Sometimes leggings were worn underneath. But there was always a second layer. No one went out in public wearing just the thong leotard. And despite what her character says in the movie ("Everybody is wearing them"), there was never any such fashion in swimming suits. There are nude beaches and there are topless beaches, but there's never been a non-nude beach (or ordinary hotel swimming pool as in the movie) where women (of any age) wore what she is wearing.

I know that there had to be a parent or guardian present on the set and that they had to sign waivers; but that is true of all movies with actors under 18, whether there is anything sexy in the movies or not. Moreover, I find it hard to believe that a parent's permission is enough to turn something that would otherwise be child porn into something legal. If my neighbor had a 14 year old child and I filmed her naked butt (and sold the tickets to the film, made a DVD, etc.) would that be legal simply because my neighbor gave me permission?

Every few months I read a story in the newspaper about somebody arrested for molesting children and almost invariably the story says that child porn was found on their computer and that they are also being charged with possession of child porn. So, it seems like anyone who owns the DVD of this movie is committing a crime.

I'm probably missing something here, but I wish someone (someone who's qualified to know, please) would tell me what it is.