I feel I've addressed this a priori:

  "Lest somebody tries to muddle the issue, I'm also not required to donate blood or organs to my two children, or my sibling, or my spouse; I should, and I likely would, but I'm not legally forced."

I get this is uncomfortable; I understand it's unpalatable; most people don't like to get into nitty gritty dilemmas. But we do not, repeat do not, require parents to donate blood or organs to their children... at least, not after they're born. Some countries do try to require it before their children are born though. Through this perspective, this feels at least backwards and misguided, and plays into the pro-choice (unfair and dishonest) stereotype meme about caring more about unborn children than born ones :-/