Population Demographics-Child Mortality Rate

How can decreasing child mortality within a country change the population pyramid? 

Well, firstly, if all the children in a country die before they can reproduce the population will reduce drastically within a few years and die off, that's only in extreme cases of course, extreme meaning ALL the children die off. However you can still apply this to a slightly smaller scale, say HALF the children die. The population will still be drastically reduced, but it will still survive if, when the children grow up, they have kids of their own and those children have children, and so on, and so on. It doesn't JUST have to be applied to death, if you have 5 kids and 4 die, it's the same as only having 1 kid to start with, as long as that 1 kid survives. 

Look at this population pyramid, it's from 2000. 

When you use the logic of child mortality you can see that if all the people from about 0-14 died, in a few years there would be very few people 0-14 anymore, because they won't have reproduced. Fewer and fewer people would be on the population pyramid. Then, eventually, none. 

So, that's basically how DECREASING child mortality rate would affect the population pyramid, INCREASING would be quite the opposite. But I won't go into that now.