If you are helping a group of people who speak a particular language, do you need to know at least a few words of that language? For example?
Unless your purpose is to teach English, then of course knowledge of the language of the country would be useful, and also show a basic respect. I would encourage you to find out as much as possible about a country before going there, whatever your purpose in visiting. You also need to think about what are you really going to achieve by going to another country. Read about "voluntourism". If however you are doing voluntary work in your own country, then the newcomers will probably already speak some English. It would still be great to learn about the people you're working with. A few words of Arabic or Spanish will be appreciated as a gesture. With the right attitude, the people can teach you do much more! We earn not only about their culture, but about our own! Good luck!
Yes, you do need to learn some basic phrases, not just words. And you should learn that if children avoid looking you in the eye, it may be part of their culture, not dishonesty on their part. Your words that concern me are that you prefer to "slip by" without knowing a "single word" of their language. It makes me wonder why you are interested in volunteering to work with children in the first place, if you are hoping not to have to learn anything new. In my opinion, you sound like a disrespectful "Lady Bountiful" and should seriously reconsider doing this. Regardless of language, children are very quick to intuit our real motives, and if those are not good ones, they will see right through you.
Well, it helps A LOT if you have a language in common with them. It doesn't matter much WHICH language that happens to be. Esperanto can be learned a lot faster than English, and doesn't have (neo-)colonialism attached to it