Martingale, this kind of an effort seems quite a bit different from the core efforts of effective altruism. Consequently, I suspect that this question won't get much traction that you probably won't get many useful suggestions. An additional factor is that volunteering in distant places for a short period of time has some complications involved in it. You can read here if you want an introduction to some of the ideas, or explore the book Ours to Explore: Privilege, Power, and the Paradox of Voluntourism if you want a more in-depth exploration. In general, I'd encourage you to have different mental buckets for "doing good" and for "travel/leisure."

However, if you are set on using your time/effort to volunteer in Argentina, I think that the subreddit for Argentina might be able to give you some suggestions. I'm guessing that most NGOs or non-profit organizations wouldn't find it a good use of their resources to provide financial help to your kids, but you can try reaching out to Habitat for Humanity in Argentina, or to organizations like International Volunteer HQ.