Unwind by Neal Shusterman (2007)

A gift from Maria on my birthday, this brought up a very disturbing idea. What if we could "unwind" the kids that we didn't like? If they didn't behave or live up to our expectations, a family could choose to "unwind" a child before their 17th birthday. Not killing them, but using their body parts for others that were ill or had lost a limb or organ. So three kids that are ready to be unwound find themselves running away and find a place for similar youngsters. At times creepy, but some unique ideas being floated.