What do you think of the idea of building spare parts for one of your kids...?? Is this ethical?
I love this topic... not because I think there is a simple answer to the question, but because I'm trying to develop topics for discussion in a bioethics course.
As usual, in these issues, it seems to me that there is a spectrum of situations to consider:
From parents who are having another child *anyway* selecting embryos such that they are a blood-type match for an existing child who frequently needs transfusions such that the new sibling could be a potential blood donor (i.e. provide tissue that is easy, nearly painless to extract and harmless to the donor), to having a child specifically to grow an organ needed for an existing child.
One thing that seems clear is that the potential donor should be at least old enough to make an informed decision for themselves before any tissue is taken, and secondly, that any such donation should be purely voluntary, regardless of how harmless it might be to the donor.
The most extreme (and obviously unethical) example of this has turned up in science fiction several times, in stories where people have themselves cloned in order to provide healthy young bodies into which their brains can be transplanted when they get old.
So the topic for discussion should be 'where do we draw the line?'
I worked on a case here in NL where a young family with a single child got some devastating news: the child has acute leukemia and the prognosis was poor without a bone marrow transplant.
A world wide search for an unrelated donor was futile, no suitable donor to be found. The mother and father were unsuitable as a donors too.
In desperation, the parents decided to have another child. The child with leukemia was in remission and healthy when the family's second child was born, a healthy baby boy.
This child was a perfect match for a bone marrow transplant. (statistically a 1 in 4 chance to be an HLA match)
When the younger brother was old enough, a transplant was performed and was a success. The children are now both healthy.
The younger child was unable to give consent because he was too young.
Did the parent's do the right thing?
Was the final outcome ethical?
This is an extremely difficult question. Bryanc has mentioned many important considerations. I think that intent has to be dismissed simply because judging the intent of people having children seems like a fool's errand. Furthermore, is it anyone's business?
From there, you have got the problem of the child's age of consent (if any) to submit to non-destructive harvesting; the pain of the procedure and more. It would be easier if brainless bags of organs could be raised but, barring that, this is just the beginning of difficult questions.
Science is going to make the moral (values, in objective terms) "challenges" faced in the 20th century look simple by comparison, even if the results are not as significant. Good. Why should things be simple?
Of course we still have not, in my opinion, properly handled assisted suicide. Sometimes, as with other difficult decisions, it seems a lot easier to not make them as voters, and to see where we end up.
That would brings up the second point. It is not anyone's business why someone wants a child.
Love, revenge, sad attempt to save a relationship, etc. The means can have implications with regards to child support payments but that is somewhat different. Aside from practicality, making intent relevant for this opens up too many problems. It becomes difficult to logically defend only considering intent with regards to this one topic while there are so many more devastating things that parents can do to their children.