Texas law doesn't yet cover artificial insemination.  Like most States their legislature is waiting to see how "large" the issue is--it isn't yet politically sensitive enough, and doesn't happen often enough to need laws.  That will change, of course.  However, Texas will likely follow the other large states (CA, FL, IL, and NY) when they get some test cases through the court system.
A "notarized" signature would mean no more than a non-notorized one, but the point is to define the specifics of: parenthood, birth expenses, child support, and long-term financial support.  If these are spelled out well, and the child never enters into the State welfare system (food stamps, childhood nutrition issues, etc.) then you'd sure be in a strong position.

That said, Judges exist because two people that sign a contract often can't agree on pretty simple things that seem to be well spelled out.