Humanism to the rescue!
The real problem is not if infanticide is morally wrong for us, it sure is (I just take this for granted, if somebody wants to try to justify it please do, could be interesting). The problem is what we can do about it. Take away the children of the Indios? Take away their culture and autonomy? That might be very cruel as well for us, it bears all kinds of other dilemma, f.e. how can we inflict this kind of terror on the mothers and fathers? What if they wage a war to defend their traditions, and get themselves killed in the dozens do we want that? Might it maybe do more harm than good to follow the precepts of those evangelicals?
The dilemma it for sure does not bear is “oh noes, it is ‘right’ for them therefore it is also right for us (or we have to ‘tolerate’ it, meaning the same)” a typical brainless excuse (based on a very obvious fallacy) of moral relativists for oppression everywhere, from FGM, witch-hunts, child brides to the stoning of rape-victims.
In short: we know that this is wrong because those precepts for one violate the Golden Rule NOWADAYS, even though those rules might have had a “evolutionary” purpose in the past, the said infanticide for example enhanced the chance of group survival for the half-nomadic Indio-tribe, the women could not carry more than one baby and the men had to carry the household items.
Now however it is possible to use birth-control (something those Evangelicals who made this movie for example did not ponder) or the possibility to put their infant in an orphanage or foster family.
I’m in favour to introduce those tribes to this possibilities. I would also guess: most of them will agree to change the traditions if it does not contradict certain power roles and if they can afford it, all humans love their children.
Especially if it is presented to them by people who speak their language and understand their traditions and do not arrogantly dismiss them as “devilish” because they think they “own” morality because some guy a long time ago was nailed on a cross.
So, does one of you want to become a Humanist missionary? 
