for volunteer teaching

we are going to Chendu as past of an international project to teachin a school for the deaf for 2 weeks.
There are 4 of us - 2 parents and our 2 sons, 18 and 20 years old, we will all be active teaqching. We will not be paid but understand we still need a working Visa. We have flights/hotel reservation, forms and photos, and they are writing us an invitation letter. Is there anything else we need. We are representing Mexico as part of this project.