The post is basing this on the assumption that having children is some positive thing for society at large. I'd argue the viewpoint that having children is putting a burden on the rest of society, and therefore doesn't necessarily need to be encouraged or subsidised, is just as valid.
I don't think benefitting society is a common reason for people to want to have kids. I don't know if there is any data on this, but certainly doubt that any of my friends with children would give a reason to that effect.
Bluntly put, each child is an additional strain on the depleting resources of an already grotesquely overpopulated planet. Ever since it no longer is an economic necessity, having children is a vanity project. I don't see why society should financially incentivise that, especially as we're trying to encourage more environmentally conscious lifestyle.