Regardless of your concerns, please DO it. There are few opportunities where children/ grandchildren request your presence and talent. As a former retired teacher, yo might want to make cut out "kits" for free or a minimal charge, which would save spending time over fabric choices and cutting instructions (those could be included in a handout that comes with the class). Also, if you can, find a sewer/quilter as a silent helper to assist in that large a group. Good luck & enjoy!