1. No politician would be brave enough to propose it even if it were a good idea.
2. The people will not support it.
3. The courts would eviscerate it, and rightly so.
4. It is so completely inconsistent with the Founder's vision of limited government power that they would puke on their velvet suits if the heard about it.
Solve those four problems, and you're golden.
Neither did the founders envision the welfare state we've spawned. Further, there's no forced confiscation here; there's a choice: They can get married and keep the kids, or give them up.
What you are forgetting when you say this is that single parents effectively give their children to the state (of course, so do many two parent families). This aspect of the design adresses the lesser of two evils: a single mother forced into a hard choice pursuant by her choice to get pregnant with the necessary result of a two parent family raising a child, or the government effectively raising a single parent kid?
I don't "like" either mechanism, but it beats gays, lesbians, single moms, easy divorce with children involved, and especially government raising kids. Let tier two have the no-fault system.