Unfortunately these people don’t have a benefits system to bail them out when unemployed or when they reach retirement age. It’s those children who die all to easily, which the elders rely on to support them in old age. Thus without numerous children they face a life of poverty. The more children they have the more likely they are to reach a level of security in later life. It is also why contraception is seen as a white mans weapon to wipe out the black man.
However, a lot of third world countries are getting a grip on fertility and family size. The benefits of this are enormous for the women, the family and the state.
Bangladesh is a good example. Their current total fertility rate is 2.1. In the 60s it was 6.6. So 2-3 kids/family on average, tending to the lower. This is a huge reduction on 7 kids/family. The reduction is primarily down to contraception, but other factors are involved.
This means the population is effectively stable (which might be a demographic timebomb primed for the future). Infant mortality is hugely reduced as is maternal mortality.
Have a look at the videos by Hans Rosling for some good news on this problem.