if their last name is thompsons then it would be thompsons'
if their last name is thompson then it would be thompson's
edit: i think.
The latter would only imply that there is one "Thompson" living there. If there are more than one of the same family there, they are the Thompsons, and if the plural is possessive, the
sign should read " Thompsons' ", as is the residence of every family member.
The door to the washroom here was marked "Mens". Drove me crazy, as "Men" is already plural, so I threw a vinyl comma between the N and S.
Edit: Of course, I should stress that " Thompsons' " is only appropriate if the intent is possessive, as if to say "The Thompsons' residence". If you want the
sign to simply mean that "The Thompsons live here", "Thompson" is fine.