I think you must have read the quoted passage but not internalised it, because it does actually say that there should be no law against killing your kid.
No, it does not say that. It says that you should not be compelled to feed your kid. If you read the passage in context, you'll find Rothbard is explicit that you cannot kill the kid, nor stop the kid from leaving if the kid is not happy with the support you are providing.
Of course as a practical matter, if the kid is young enough, or you live sufficiently far from other people, the kid cannot exercise its right to leave, and so effectively you can kill it by not providing food.
