Quattro - I did answer your question.
"If two cells is okay ... then what is wrong with bone marrow transplants ..."
As I said before, courts routinely authorise bone marrow transplantations from siblings who are too young to properly consent. I don't see why you are bringing it up as if it is to be considered for the first time because of advances in genome technology.
I am not a lawyer nor a doctor, but my understanding is that the courts agree with this with the view that the minor inconvenience of the operation on the child is outweighed by having a living sibling.
I asked you before to look up the ethical considerations if you are so interested, and you still can.
"if bone marrow transplants are okay ... then what is wrong with organ donation ..."
Again, not much. As I said before, in absolutely necessary cases, courts have authorised children donating organs to their siblings.
"if organ donation is okay ... then what is wrong with ultimately sacrificing the saviour child's life"
This is a logical fallacy called "Slippery slope" ('Non causa pro causa' iirc). You are putting forth the false assumption that bone marrow donation and organ donation would ultimately lead to killing the child to get his organs. Of course it won't. The goal here is to save a life (the sibling's) not to take one (the donor's).
Your argument is akin to saying "If it's ok to file nails, what is wrong with cutting cuticles?". "And if it's ok to cut cuticles, what is wrong with cutting a finger". It is of course a false argument, since the first two (filing & cutting cuticles) are, although possibly unpleasant at the time, done with the ultimate goal of having prettier hands and would in no circumstances lead to cutting off your fingers.