I agree with Peter, that the right to have children -- or, as I prefer to frame it, the right to procreate -- is not a positive claim right. No one has the correlative duty to supply you with children, or even (I'd argue) with the means to have children. But it's a fascinating question. My own thinking is that there is a tendency to think of the right to procreate as a right of property or acquisition, when in fact it is a right of expression. Procreation is an activity (and as Peter points out, it is an activity that does not guarantee a product), and it is this activity the chance for which is ordinarily considered to be so fundamental to living a good human life that it is protected by a right.