If you will be staying with a host family on your volunteering trip abroad, it is a kind idea to bring a usable and practical gift.
Some examples include pretty soaps or candles, lotions, nice picture frames, facial cleansers, Advil / Tylenol, and nice pens.
An even better idea is to buy something practical for the family after you have stayed with them and gotten to know their needs (ie a new rice cooker).
We do not recommend sweets/chocolate, whether for your host family or the organization where you will work, because few people see dentists especially children.