Umm:idea:
if you just have to take a photo of your display for your assignment, surely the children in any small photos won't be clearly visible - so I wouldn't worry about this. I always think that the best displays have photos of the children creating them so do make sur you include them.
As for an idea for the theme...
How about getting the children to dress up as various 'people who help us' - e.g. police officers, fire fighters (very PC speak!!) ... and create a collage of one (draw round one of the children and decorate together?) ... and add some of the children's remarks about these people and how they help us in speech bubbles on the display... combine with a visit from one of these people if appropriate
Hope that helps a bit!