Learn the Bengali alphabet. This is a good first step to learning any language. The Bengali language uses a different character set, and even has entirely different sounds, than the Latin alphabet with which we are all familiar. But memorizing each letter and its corresponding sound is necessary to being able to read, write, and speak.
Practice pronunciation. The vowel sounds in Bengali can be particularly tough, so have the children practice these over and over, and out loud. They may get great enjoyment from hearing themselves and their peers try to vocalize such unfamiliar sounds. Perform listening and repetition exercises until the children have a strong grasp of how the letters, phonemes, and words are formed.
Watch videos in Bengali. While the children won't be able to understand the language right away, watching film will help them internalize the sounds and provide a vivid look at the culture surrounding the language. If possible, turn English subtitles on to help them follow along with the words. You can use Bengali DVD's or online Bengali video libraries (see Resources).
Make a game of it. Use familiar child's games, like Hangman, to teach the language. Use short, simple words and have the students guess the letters one by one, per the standard rules of Hangman. The entertainment factor will encourage them to learn, practice, and guess, putting their knowledge to the test.
Use pictorial resources for vocabulary lessons. Develop cards featuring simple nouns (such as "tiger" or "boat"), the Bengali word, and the English transliteration. They will connect the picture with the Bengali word and learn how to pronounce it from the English transliteration. Practice repeatedly until children develop a basic vocabulary. This activity can be done individually or in a group.