I realize this is a made-up scenario, but Westerners tend to assume it's irrational for those in poor countries to maximize their number of offspring. There's a number of factors (labor value tradeoffs, insurance against periods of inability to sustain sufficient income esp at end-of-life) that militate otherwise. Minus could result in a huge amounts of unintended disutility, particularly as populations age.

As pointed out above, the best option is to move the poor into situations where institutions are stronger, producing higher incomes. However, this is very difficult on any large scale. For instance, moving the entire population of Africa to Norway (ignoring space constraints) probably wouldn't increase overall incomes because local institutions would be overwhelmed by new voter preferences and cultural norms. Unfortunately, it's also extremely difficult to develop these institutions in poor countries (the incentives tend to point the wrong way, see Acemoglu on the "iron law of oligarchy").

So the best donation bet might be "meta-Growth" -- attempts to increase the value of Growth by researching more effective methods of intervention to increase Growth.