What a wonderful experience! You'll be able to make a positive impact on their lives. :)
Personally, I would start with the things that they will need to do in their daily life, as they start to speak English. For example, shopping at a supermarket (buying and selling, prices, asking if they have such-and-such, etc.) I'd also focus almost exclusively on communication, since they are already in the country. I'd aim to make the classes immediately useful, to keep them coming back in the beginning. There are things that they avoid doing because of the language barrier. What are they? It might be nice if you could do a simple needs analysis with them to find out what they wish they could do, and what kind of language learning background they have. Maybe you could develop a survey they can fill out, all in Spanish, at the beginning? Even if people come and go, the results might give you some idea of where to start and which direction to head in.