Currently, most of the poor countries live in rural areas, where the major source of income comes from farming. This work is very labor-intensive, and having more children means there are more hands for work. Once the population becomes more educated, the skills it gains can be used in other sectors. But if there is no infrastructure to make use of these skills in local areas, the population will start migrating to more developed cities, stretching the infrastructure and resources there.
Perhaps now you can notice the link with Sustainable Development Goals 4, 8 and 9, which are quality education, decent work, and economic growth, and industry, innovation, and infrastructure respectively. Educated population (goal 4) needs to have access to decent work to ensure economic growth (goal 8), for which they need industries and infrastructure development on local levels (goal 9). For all of this to take place on the local level, rural areas need to be urbanized.