Here's a little more about their folk music:

Most prevalent of folk songs and music traditions include Bhatiali, Baul, Marfati, Murshidi and Bhawaiya. Lyricists like Lalon Shah, Hason Raja, Kangal Harinath, Romesh Shill, Abbas Uddin and many unknown anonymous lyricsts have enriched the tradition of folk songs of Bangladesh.

Lots of Bengali songs recorded in the field in Bangladesh: click, again all in their native language.

Getting closer, here are some kids' songs in Bangla and English, but no tunes! Click.

There are some neat sound files in THIS.

Have fun and congrats on the gig!