The economic benefits are already balanced in favour of not having children, hence the falling birthrates. Taxation won't do very much to change this, either way. I also don't like the idea of the Government getting involved in areas such as this, this is the USA, not China.
What's more, if your tax was as successful as hoped, the economy will yearn for a greater working force down the line, which will mean that it'd require an influx of immigrants. This can cause a lot of disturbance amongst the current residents, and can lead to the rise of parties and groups that aren't of the, let's just say, best tastes.







