I think mattbateman's approach is good, look at the facts:
I would add:
3. A child is different from a dog or a car. And the acquisition of a child, either by making one or adopting one, entails a different set of responsibilities. The difference is that a child is a human, and has rights. And as such, has the right to freedom from physical force or coercion.
4. A child is not competent to judge his best interests, and cannot and should not be expected to.
By having a child, a couple does indeed take on the responsibility to raise that child to the best of their abilities until that child is self-sufficient. I would say that morally, parents have the obligation to do their absolute best for the child. It is part of taking full responsibility for your actions. As Matt points out, the child had no choice in the matter. The parents did. So the parents are accountable. The correct attitude of parents towards children should be: "I know you didn't choose life, we chose it for you, so we are going to do our best to make it the best it possibly can be for you."
As far as the role of government goes, I would say this: A child has rights. The government's job is to protect those rights. By being, by definition, dependent, it is indirect force to deny a child sustenance. With regard to what extent the government should get involved in judging what qualifies as "sustenance", I think the line is at the basic requirements for physical survival. This is food, shelter, clothing, etc. Failure to provide these things to a child at a basic level would qualify as negligence and a form of child abuse. It would then be the responsibility of a wing of police departments to investigate and prosecute instances of this. Judging what and what is not basic sustenance for a child is a judgement call that would have to be made by a qualified judge with all the relevant facts of the society, level of technology, economy, etc, at his disposal.
With regard to at what age a person becomes an adult, I'll share one of the best ideas I've ever heard. The idea is that within a specific age range, say 15-21, a young adult can go to a government office and sign a paper that legally declares him of the age of consent. Once that paper is signed, he is legally an adult and entitled to all the privileges of adulthood (voting, ability to enter contracts, marriage, etc.) At the same time, if he then commits a crime, he is prosecuted as an adult. The nice thing is the person then gets to assume the responsibilities of adulthood, when ready. It also gets the government out of the business of declaring these arbitrary points in people's lives where they can and cannot do certain things.
Ben
http://www.gmuoc.org