"Those last two rights are those that the child wouldn’t really be able to enjoy anyway. No child can legally own property, and with curfews and other restrictions unique to children they can’t really be said to enjoy liberty in the same way that an adult does. Why shouldn’t life, like these other two rights, be a privilege based upon the child’s development?"
Of course the unborn child won't be enjoy them, if you abort him/her. Your concept of life as a "privilege" to be earned implies that someone else - the expectant mother, the courts, some federal agency, etc. - gets to grant or withhold the "priviledge" of life that priviledge based upon some arbitrary decision that "he/she wouldn't have enjoyed life anyway."
Abortion isn't about where you draw the line or murder - at conception, implantation, first second or third trimester, or beyond - it's about granting some members of the human race the ability to fatally decide whether other's lives are worth living at all. How is that different from murder?
There already exist circumstances under which some members of the human race (government, the court) get to decide whether other's lives are worth living. This is just adding another.
This isn't exactly a new concept either. Death has traditionaly been imposed upon persons for relatively benign offenses and even deformity.
I would hasten to add that in this case, the child's mother is the one who gets to make the decision, which is hardly a impersonal committee. I also grant it is not a decision to be made rashly, and every opportunity to persuade the mother to keep the child should be used, including providing sonograms and other educational materials.