Well, first and foremost, consider the issue of family planning: there is no doubt developing countries need to lower their birth rates dramatically to reduce the social, environmental and economic burden of providing education, healthcare, food, water and jobs amongst other things, to so many people. For sure, some of the poor in those countries understand this and want to have less children but cannot, for reasons including sexual pressure from males, lack of access to (or awareness of) contraceptives and reproductive health knowledge.
But, for many they are not aware of the impacts their children will have: the impacts on the child, on the parents, on the wider family, on the country, or on the environment. They have not done the calculations: if they have they probably still believe that having more children helps provide for the family in the medium-term, and for their parents in the long-term. Overlooked is the real short-term needs as well as a sophisticated understanding of longer-term impacts from splitting land up between children for example. Of course it is on the larger-scale where most of the problems lie, and this falls to the government to deal with (i.e. in providing social services, employment, food etc).
Thus it is hugely important for governments to take the lead, for their own self-interest, in in-expensive education and awareness campaigns for their citizens to better understand the impacts their children might have. Without creating this desire, through role-models, peer pressure, media, peer-to-peer education and so forth (varied and thought-out campaigns utilising all available channels), the poor may never even want to limit their family size.
Once such a desire is created, then specific education on family planning and contraception can be provided along with increased access to a variety of contraceptions and other longer-term social activities to address larger issues such as coerced sex, female empowerment, poverty etc.
