Well, your relatives are probably right depending on how a "family" is defined.

Are we talking about children per household, do multi-generational families count? Then they're definitely more right than not.

Or are you all fumbling your way towards discussing the "birth rate", which is how it's usually discussed. That may or may not be the same as "kids per family".

No, it is all about the birthrate. Those countries are poor as "they are having more children than they can feed, yet alone afford to educate."