Ordinarily, I'd be annoyed that people take "this will happen" to mean, "I want this to happen." But, hey, you got it right this time!
There's a very specific reason I referred to child abuse, and not some other crime. I could have also used the example of taking one's wife to a state where it's legal to have her forcibly sterilized. These are situations where people could commit serious crimes against members of their own families, w/o consequences, by simply taking them to states where horrific things are legal. Not much comparison there, to buying magazines or fireworks, or whatever. Mala in se, etc.
Are we OK with having The Purge states, where anyone can go to commit egregious crimes? This is far from hypothetical. What does Texas do if someone takes a child across state lines to "transition" them, and brings them back (because it's child abuse there now)? What did the U.S. do about slaves crossing state lines to become free? See the fugitive slave laws, and the Dred Scott decision.
But, like I said, even if it's perfectly reasonable to prosecute people for taking their children out of state to kill them, that doesn't mean it can be enforced.