Up to 18 months, all you can do is play with them in English. Don't expect to get them all doing the same thing, just have music playing all the time and a selection of books, flashcards and games out and go around interacting with them. Speak to them constantly. Try to get them to repeat, sing along, etc, but probably there will be zero response in English for at least the first couple of weeks. You'll need to understand and praise correct answers in L1 to your questions in English, so if you don't speak the students' L1 get a list of things they are likely to say and study that.