We agree on a lot. We agree that educational vouchers would "help poor kids get a decent education" and that this is a worthy objective. (We disagree about whether the Constitution stands in the way.)
We agree that it is constitutional to give churches police and fire protection, paid for by tax dollars--though we may not agree on why. I say it is because these benefits are provided neutrally to all citizens, without favoring religious over secular recipients. That is the core of the Establishment Clause: The government should neither favor nor disfavor religion. You say that the government cannot "make us pay taxes to be used for religious indoctrination," but that does not answer the question. When the government pays for schools, it is not using the funds for "religious indoctrination." It is using the funds for education and leaves to private choice whether that education is to be from a secular or a religious point of view.
We agree that it would be unconstitutional to have a National Endowment for Prayer. But this is because the National Endowment for Prayer would not be neutral: It would subsidize an activity that is religious, and only religious.
I am glad you brought up the analogy to the National Endowment for the Arts. I assume you agree that the National Endowment for the Arts can give grants to artists who produce religious art, as long as the grants are made on the basis of artistic merit. Stained-glass windows could be funded, or performances of Bach's Mass in D Minor. When you and I served together on the independent commission to advise Congress on the NEA funding issue, you defended the right of an artist to receive a grant to depict a crucifix dipped in urine. To be consistent, you must surely agree that artists have an equal right to produce art treating religious symbols with reverence or respect. Religious art can be just as beautiful as nonreligious art, and NEA grants ought to be given on the basis of artistic merit and not ideological content.
The same is true of schools. Religious schools can be just as good as secular schools. (Indeed, in the inner cities they are often better.) To exclude excellent schools because they are religious is not neutral--it is hostile--toward religion.
You profess not to understand why I think "a decision to withhold a subsidy amounts to 'hostility.' " You say that "leaving religious schools to flourish on their own, with private funds" is not "hostility." But when Jesse Helms said that "indecent" artists should be denied funds, you called it a penalty and said it was unconstitutional. You criticize parents who think their children should get an "equal share" of education funds. But denying people their "equal share" of a generally available benefit is what we generally mean by discrimination. I'm sure you'd perceive the hostility if funds were denied to atheist schools, on the ground that they are "indoctrinating" students in their ideology. What's the difference?