1. It is good to have children per se.
2. Demographic decline per se is not in the interest of the state or society.
3. It is in the interest of the state to create incentives for marrying and having children and to support or defend a culture that is at least not hostile to family life and its flourishing.
Does it follows that companies are obligated to pay a higher salary to someone with a family over someone who is single for the same work? If labor is the source of economic value, then it would seem that they are not since the married person with children is creating the same value.
Of course, when employers do compete for employees, such candidates will have a greater need and incentive to choose the employer who does pay more, and companies, realizing that such people will find better offers more attractive, may provide benefits that specifically target such people (e.g., free daycare or whatever). Whether companies do this will depend on market conditions, for example.