"We want good sandwiches."
Translation:Queremos buenos sándwiches..
My Spanish-speaking husband says the adjective usually comes after the noun, but it is understandable (and usually correct) both before and after the noun. A white house can be 'una casa blanca' or 'una blanca casa, but "The White House" in Washington, DC is always "La Casa Blanca"
While in Spanish the adjective is usually after the noun, it often is before the noun for some common adjectives like bueno (as in buenos días). The connotation or emphasis can be different, such as "un gran hombre" (a great man) or "un hombre grande" (a large man). In this case, I think they are looking for the more common or natural form, even though your answer might also be correct.
It shouldn't be considered wrong to use nosostros, but it isn't necessary. The only information you're adding to the sentence is that there is at least one male in your group. Although, I suppose it makes your sentence clearer, like if you're at a drive through and it's hard to understand.